abrt-retrace-client: improve error messages

Looked through abrt-retrace-client and related source files,
eyeing specifically error handling and messages.

The largest improvement comes from emitting URL
in calls to alert_connection_error(url), alert_server_error(url).
Also added URL printing to a few other messages.

Made some messages more precise:
s:"Cannot open temporary file":"Can't create temporary file in /tmp":

Added error check for tar and xz exit code.

Used xstat() where appropriate.

Removed a few PR_Close()s immediately before error_msg_and_die().
They were _sometimes_ present. No point in being inconsistent.

Signed-off-by: Denys Vlasenko <dvlasenk@redhat.com>