debugfs: abort reading a file on failure when dumping out a file

debugfs: abort reading a file on failure when dumping out a file

If ext2fs_file_read() fails --- perhaps due to a corrupted file
system, or an I/O error --- avoid looping forever in dump_file().

This issue was pointed out in [1] by Quentin Kaiser but the commit
description was too confusing and specific to the user's particular
situation.

[1] https://github.com/tytso/e2fsprogs/pull/149

Signed-off-by: Theodore Ts'o <tytso@mit.edu>
(cherry picked from commit 51d68472456d22b6e64159244be63bce51473691)