Add file_error() calls after file_read().

If file_read() doesn't return the exact number of bytes you asked for,
and you really need all those bytes to be there, you have to call
file_error() to find out what the problem is.

Change-Id: I4cc87bc1b6cc5a49bbcbf93b56106f57af290d20
Reviewed-on: https://code.wireshark.org/review/4205
Reviewed-by: Guy Harris <guy@alum.mit.edu>