Replace 'error' by 'warning' when a signal cannot be translated to the gdb nr.

Replace 'error' by 'warning' when a signal cannot be translated to the gdb nr.

As 'error' is supposed to be called only to go back at gdbserver toplevel,
and such signal translations can be called during tracing, when
gdbserver is not active : 'error' then tries to longjmp using a
not initialised longjmp buffer.

(reproduced by activating the trace with the test case attached in
bug 409141 - Valgrind hangs when SIGKILLed).