Improve SyncFS error messages for File Errors.

This probably won't help much in several error cases (especially for generic
'operation failed' case), but would still give slightly more friendly feeling.

(I'll also look into how requestFileSystem or getFile could return very
unhelpful 'PLATFORM_FILE_ERROR_FAILED' error)

BUG=325591
R=nhiroki@chromium.org

Review URL: https://codereview.chromium.org/103783002

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@238645 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 238645
Be cautious and cancel modal dialogs before deleting any RenderViewHosts.

BUG=324320
TEST=None

Review URL: https://codereview.chromium.org/103243003

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@238646 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 238646