Remove annoying 'no differences' error dialog

When the user selects two identical patch sets, the server throws
a NoDifferencesException, which causes Gerrit to display an error
dialog.  This dialog blocks subsequent attempts to view that file
until the patch set being compared gets reset.

This change deletes the NoDifferencesException class and instead
displays the message "No differences" in the diff table if the user
selects two identical patch sets to diff.
4 files changed