afh_recv: Improve error diagnostics.

afh_recv: Improve error diagnostics.

There are many reasons for afh_recv_open() to fail. For example, the
afh receiver could be unable to open its input file, or the given
begin chunk was larger than the end chunk. At the moment the error
reporting is a bit scarce since only the string of the error code is
printed. This commit makes afh_recv_open() print also the reason for
the error and the name of the file that caused the error.