[preferences][enhancement] Provide a better error message when the user/workspace file is not a valid JSON

Description

Reproduction Steps

• Manually edit one of the settings.json file. Make it an invalid JSON.
• Restart the frontend app.

User error:

The user might not know what the user_storage:settings.json means.

Error in the browser console:

root ERROR Failed to update the value of workbench.iconTheme. Error: Can not add index to parent of type property
log @ logger-protocol.ts:112
(anonymous) @ logger-frontend-module.ts:41
(anonymous) @ logger.ts:312
(anonymous) @ logger.ts:304
Promise.then (async)
../node_modules/@theia/core/lib/common/logger.js.Logger.log @ logger.ts:299
log @ logger.ts:45
(anonymous) @ abstract-resource-preference-provider.ts:116
step @ abstract-resource-preference-provider.ts:15
(anonymous) @ abstract-resource-preference-provider.ts:15
fulfilled @ abstract-resource-preference-provider.ts:15
Promise.then (async)
step @ abstract-resource-preference-provider.ts:15
fulfilled @ abstract-resource-preference-provider.ts:15
Promise.then (async)
step @ abstract-resource-preference-provider.ts:15
(anonymous) @ abstract-resource-preference-provider.ts:15
push.../node_modules/@theia/preferences/lib/browser/abstract-resource-preference-provider.js.__awaiter @ abstract-resource-preference-provider.ts:15
push.../node_modules/@theia/preferences/lib/browser/abstract-resource-preference-provider.js.AbstractResourcePreferenceProvider.setPreference @ abstract-resource-preference-provider.ts:89
(anonymous) @ preference-service.ts:260
step @ preference-service.ts:15
(anonymous) @ preference-service.ts:15
(anonymous) @ preference-service.ts:15
../node_modules/@theia/core/lib/browser/preferences/preference-service.js.__awaiter @ preference-service.ts:15
../node_modules/@theia/core/lib/browser/preferences/preference-service.js.PreferenceServiceImpl.set @ preference-service.ts:251
../node_modules/@theia/core/lib/browser/common-frontend-contribution.js.CommonFrontendContribution.updateThemePreference @ common-frontend-contribution.ts:315
(anonymous) @ common-frontend-contribution.ts:297
(anonymous) @ event.ts:117
../node_modules/@theia/core/lib/common/event.js.CallbackList.invoke @ event.ts:125
../node_modules/@theia/core/lib/common/event.js.Emitter.fire @ event.ts:217
set @ icon-theme-service.ts:172
../node_modules/@theia/core/lib/browser/icon-theme-contribution.js.DefaultFileIconThemeContribution.registerIconThemes @ icon-theme-contribution.ts:56
(anonymous) @ icon-theme-contribution.ts:40
step @ icon-theme-contribution.ts:15
(anonymous) @ icon-theme-contribution.ts:15
(anonymous) @ icon-theme-contribution.ts:15
../node_modules/@theia/core/lib/browser/icon-theme-contribution.js.__awaiter @ icon-theme-contribution.ts:15
../node_modules/@theia/core/lib/browser/icon-theme-contribution.js.IconThemeApplicationContribution.onStart @ icon-theme-contribution.ts:38
(anonymous) @ frontend-application.ts:335
(anonymous) @ frontend-application.ts:365
step @ frontend-application.ts:15
(anonymous) @ frontend-application.ts:15
(anonymous) @ frontend-application.ts:15
../node_modules/@theia/core/lib/browser/frontend-application.js.__awaiter @ frontend-application.ts:15
../node_modules/@theia/core/lib/browser/frontend-application.js.FrontendApplication.measure @ frontend-application.ts:361
(anonymous) @ frontend-application.ts:334
step @ frontend-application.ts:15
(anonymous) @ frontend-application.ts:15
step @ frontend-application.ts:15
(anonymous) @ frontend-application.ts:15
fulfilled @ frontend-application.ts:15
Promise.then (async)
step @ frontend-application.ts:15
(anonymous) @ frontend-application.ts:15
../node_modules/@theia/core/lib/browser/frontend-application.js.__awaiter @ frontend-application.ts:15
../node_modules/@theia/core/lib/browser/frontend-application.js.FrontendApplication.startContributions @ frontend-application.ts:302
(anonymous) @ frontend-application.ts:123
step @ frontend-application.ts:15
(anonymous) @ frontend-application.ts:15
(anonymous) @ frontend-application.ts:15
../node_modules/@theia/core/lib/browser/frontend-application.js.__awaiter @ frontend-application.ts:15
../node_modules/@theia/core/lib/browser/frontend-application.js.FrontendApplication.start @ frontend-application.ts:122
start @ index.js:38
Promise.then (async)
./src-gen/frontend/index.js @ index.js:76
__webpack_require__ @ bootstrap:63
(anonymous) @ bootstrap:198
(anonymous) @ bootstrap:198
Show 18 more frames
preference-service.ts:263 Uncaught (in promise) Error: Unable to write to Default Settings.
    at PreferenceServiceImpl.<anonymous> (preference-service.ts:263)
    at step (preference-service.ts:15)
    at Object.next (preference-service.ts:15)
    at fulfilled (preference-service.ts:15)

OS and Theia version:

Diagnostics: