f40b7ae6ac didn't help with people who've already upgraded, so check at
startup and show a warning if the profile is inaccessible until 1) the
profile has been accessed once or 2) the user checks "Don't show again"
in the warning dialog.
Also fix Zotero.Profile.getDefaultInProfilesDir() to properly throw an
error if it can't access the default directory.
10a5d8d02e