Improve error reporting when handling SystemTap SDT probes

Improve error reporting when handling SystemTap SDT probes

This patch improves the error reporting when handling SystemTap SDT
probes.  "Handling", in this case, mostly means "parsing".

On gdb/probe.h, only trivial changes on functions' comments in order
to explicitly mention that some of them can throw exceptions.  This is
just to make the API a bit more clear.

On gdb/stap-probe.c, I have s/internal_error/error/ on two functions
that are responsible for parsing specific bits of the probes'
arguments: stap_get_opcode and stap_get_expected_argument_type.  It is
not correct to call internal_error on such situations because it is
not really GDB's fault if the probes have malformed arguments.  I also
improved the error reported on stap_get_expected_argument_type by also
including the probe name on it.

Aside from that, and perhaps most importantly, I added a check on
stap_get_arg to make sure that we don't try to extract an argument
from a probe that has no arguments.  This check issues an
internal_error, because it really means that GDB is doing something it
shouldn't.

Although it can be considered almost trivial, and despite the fact
that I am the maintainer for this part of the code, I am posting this
patch for review.  I will wait a few days, and if nobody has anything
to say, I will go ahead and push it.

gdb/ChangeLog:
2015-09-01  Sergio Durigan Junior  <sergiodj@redhat.com>

	* probe.h (struct probe_ops) <get_probe_argument_count,
	evaluate_probe_argument, enable_probe, disable_probe>: Mention in
	the comment that the function can throw an exception.
	(get_probe_argument_count): Likewise.
	(evaluate_probe_argument): Likewise.
	* stap-probe.c (stap_get_opcode): Call error instead of
	internal_error.
	(stap_get_expected_argument_type): Likewise.  Add argument
	'probe'.  Improve error message by mentioning the probe's name.
	(stap_parse_probe_arguments): Adjust call to
	stap_get_expected_argument_type.
	(stap_get_arg): Add comment.  Assert that 'probe->args_parsed' is
	not zero.  Call internal_error if GDB requests an argument but the
	probe has no arguments.