gdbserver: fix silent error exit

gdbserver: fix silent error exit

Running gdb.threads/process-dies-while-handling-bp.exp against
gdbserver sometimes FAILs because GDBserver drops the connection, but
the logs leave no clue on what the reason could be.  Running manually
a few times, I saw the same:

 $  ./gdbserver/gdbserver --multi :9999 testsuite/gdb.threads/process-dies-while-handling-bp
 Process testsuite/gdb.threads/process-dies-while-handling-bp created; pid = 12766
 Listening on port 9999
 Remote debugging from host 127.0.0.1
 Listening on port 9999

 Child exited with status 0

 Child exited with status 0

What happened is that an exception escaped and gdbserver reopened the
connection, which led to that second "Listening on port 9999" output.

The error was a failure to access registers from a now-dead thread.
The exception probably shouldn't have escaped here, but meanwhile,
this at least makes the issue less mysterious.

Tested on x86_64 Fedora 20.

gdb/gdbserver/ChangeLog:
2015-08-06  Pedro Alves  <palves@redhat.com>

	* server.c (captured_main): On error, print the exception message
	to stderr, and if run_once is set, throw a quit.