Files.app: Show error messages for the case the source file is not exists.

chrome/app/chromeos_strings.grdp

 <?xml version="1.0" encoding="utf-8"?>
 <!-- ChromeOS-specific strings (included from generated_resources.grd).
      Everything in this file is wrapped in <if expr="pp_ifdef('chromeos')">. -->
 <grit-part>
   <message name="IDS_EMPTY_STRING" desc="Empty string, exist only to make code generic. No translation required.">
   </message>
   <!-- Idle logout dialog for retail mode -->
   <message name="IDS_IDLE_LOGOUT_TITLE" desc="Dialog title for the idle logout dialog. Used in retail mode only.">
     Are you still there?
   </message>
@@ skipping 446 matching lines @@
     Share
   </message>
 
   <message name="IDS_FILE_BROWSER_COPY_FILE_NAME" desc="File Manager status message.">
     Copying <ph name="FILE_NAME">$1<ex>movie.avi</ex></ph>...
   </message>
   <message name="IDS_FILE_BROWSER_COPY_ITEMS_REMAINING" desc="File Manager status message. 'Item' is used here as a generic term for file or directory.">
     Copying $1 items...
   </message>
   <message name="IDS_FILE_BROWSER_COPY_TARGET_EXISTS_ERROR" desc="File Manager error message. 'Item' is used here as a generic term for file or directory.">
-    Copy operation failed, item exists: "$1"
+    Whoops, <ph name="FILE_NAME">$1<ex>movie.avi</ex></ph> already exists. Rename it and try again.
+  </message>
+  <message name="IDS_FILE_BROWSER_COPY_SOURCE_NOT_FOUND_ERROR" desc="File Manager error message. 'Item' is used here as a generic term for file or directory.">
+    Whoops, <ph name="FILE_NAME">$1<ex>movie.avi</ex></ph> no longer exists.
   </message>
   <message name="IDS_FILE_BROWSER_COPY_FILESYSTEM_ERROR" desc="File Manager error message.">
     Copy operation failed. <ph name="ERROR_MESSAGE">$1<ex>An error occurred (code: ABORT)</ex></ph>
   </message>
   <message name="IDS_FILE_BROWSER_COPY_UNEXPECTED_ERROR" desc="File Manager error message.">
     Copy operation failed, unexpected error: $1
   </message>
   <message name="IDS_FILE_BROWSER_MOVE_FILE_NAME" desc="File Manager status message.">
     Moving <ph name="FILE_NAME">$1<ex>movie.avi</ex></ph>...
   </message>
   <message name="IDS_FILE_BROWSER_MOVE_ITEMS_REMAINING" desc="File Manager status message. 'Item' is used here as a generic term for file or directory.">
     Moving $1 items...
   </message>
   <message name="IDS_FILE_BROWSER_MOVE_TARGET_EXISTS_ERROR" desc="File Manager error message. 'Item' is used here as a generic term for file or directory.">
-    Move failed, item exists: "$1"
+    Whoops, <ph name="FILE_NAME">$1<ex>movie.avi</ex></ph> already exists. Rename it and try again.
+  </message>
+  <message name="IDS_FILE_BROWSER_MOVE_SOURCE_NOT_FOUND_ERROR" desc="File Manager error message. 'Item' is used here as a generic term for file or directory.">
+    Whoops, <ph name="FILE_NAME">$1<ex>movie.avi</ex></ph> no longer exists.
   </message>
   <message name="IDS_FILE_BROWSER_MOVE_FILESYSTEM_ERROR" desc="File Manager error message.">
     Move failed. <ph name="ERROR_MESSAGE">$1<ex>An error occurred (code: ABORT)</ex></ph>
   </message>
   <message name="IDS_FILE_BROWSER_MOVE_UNEXPECTED_ERROR" desc="File Manager error message.">
     Move failed, unexpected error: $1
   </message>
   <message name="IDS_FILE_BROWSER_ZIP_FILE_NAME" desc="File Manager status message.">
     Zipping <ph name="FILE_NAME">$1<ex>movie.avi</ex></ph>...
   </message>
@@ skipping 4448 matching lines @@
     We see that you've ordered a replacement charger.
   </message>
   <message name="IDS_CHARGER_REPLACEMENT_CHARGER_MORE_INFO" desc="Text for fining more info for charger">
     You can find more info at
   </message>
   <message name="IDS_CHARGER_REPLACEMENT_DIALOG_TITLE" desc="Title of the charger replacement UI dialg">
     Check charger
   </message>
 
 </grit-part>