Improve messaging for provided volume unmount failures.

Bug: 795167
Change-Id: I4223e26d23885c350337aefb8abd991427435a05
Reviewed-on: https://chromium-review.googlesource.com/828184
Commit-Queue: Tatsuhisa Yamaguchi <yamaguchi@chromium.org>
Reviewed-by: Naoki Fukino <fukino@chromium.org>
Cr-Commit-Position: refs/heads/master@{#542058}

chrome/app/chromeos_strings.grdp

@@ -322,10 +322,10 @@
    <ph name="LINE_BREAKS">&lt;br/&gt;&lt;br/&gt;</ph>
    Removing your device while it is in use may cause data loss.  Please wait until the operation is over, then eject the device using the Files app.
   </message>
-  <message name="IDS_FILE_BROWSER_UNMOUNT_PROVIDED_FAILED" desc="Title of the error dialog when unmount operation for a volume provided by an extension failed.">
-   <ph name="BEGIN_BOLD">&lt;b&gt;</ph>Cannot unmount volume<ph name="END_BOLD">&lt;/b&gt;</ph>
+  <message name="IDS_FILE_BROWSER_UNMOUNT_PROVIDED_FAILED" desc="Title of the error dialog when unmount operation for a volume provided by an extension failed. One of the most typical use case is an archive (i.e. ZIP) file mounted as a virtual disk. However, arbitrary Chrome apps can mount volumes. So we should not assume this is an archive file.">
+   <ph name="BEGIN_BOLD">&lt;b&gt;</ph>Can't disconnect from an archive or virtual disk<ph name="END_BOLD">&lt;/b&gt;</ph>
    <ph name="LINE_BREAKS">&lt;br/&gt;&lt;br/&gt;</ph>
-   The volume is busy. Please close all the opened files from it, then try again later.
+   Close all files on the archive or virtual disk, then try again.
   </message>
   <message name="IDS_FILE_BROWSER_MOUNT_ARCHIVE" desc="Title of the action for mounting an archive file to work with its contents.">
     Open