[gdb, c++] Improve error message when using libstdcxx without SDT probes

When using catch catch/rethrow/catch, a libstdcxx with SDT probes is required
for both the regexp argument, and the convenience variable $_exception (
https://sourceware.org/gdb/current/onlinedocs/gdb/Set-Catchpoints.html ).

Currently, when using these features with a libstdcxx without SDT probes, we
get the cryptic error message:
...
not stopped at a C++ exception catchpoint
...

Improve this by instead emitting the more helpful:
...
did not find exception probe (does libstdcxx have SDT probes?)
...

Tested on x86_64-linux.

gdb/ChangeLog:

2019-08-26  Tom de Vries  <tdevries@suse.de>

	PR c++/24852
	* break-catch-throw.c (fetch_probe_arguments): Improve error mesage
	when pc_probe.prob == NULL.

gdb/testsuite/ChangeLog:

2019-08-26  Tom de Vries  <tdevries@suse.de>

	PR c++/24852
	* gdb.cp/no-libstdcxx-probe.exp: New test.

gdb/ChangeLog

@@ -1,3 +1,9 @@
+2019-08-26  Tom de Vries  <tdevries@suse.de>
+
+	PR c++/24852
+	* break-catch-throw.c (fetch_probe_arguments): Improve error mesage
+	when pc_probe.prob == NULL.
+
 2019-08-25  Simon Marchi  <simon.marchi@efficios.com>
 
 	* dwarf2read.c (dw2_debug_names_iterator::next): Rename local

gdb/break-catch-throw.c

@@ -106,8 +106,10 @@ fetch_probe_arguments (struct value **arg0, struct value **arg1)
   unsigned n_args;
 
   pc_probe = find_probe_by_pc (pc);
-  if (pc_probe.prob == NULL
-      || pc_probe.prob->get_provider () != "libstdcxx"
+  if (pc_probe.prob == NULL)
+    error (_("did not find exception probe (does libstdcxx have SDT probes?)"));
+
+  if (pc_probe.prob->get_provider () != "libstdcxx"
       || (pc_probe.prob->get_name () != "catch"
 	  && pc_probe.prob->get_name () != "throw"
 	  && pc_probe.prob->get_name () != "rethrow"))

gdb/testsuite/ChangeLog

@@ -1,3 +1,8 @@
+2019-08-26  Tom de Vries  <tdevries@suse.de>
+
+	PR c++/24852
+	* gdb.cp/no-libstdcxx-probe.exp: New test.
+
 2019-08-26  Tom de Vries  <tdevries@suse.de>
 
 	* lib/gdb.exp (skip_libstdcxx_probe_tests_prompt): Return 1 if probe

gdb/testsuite/gdb.cp/no-libstdcxx-probe.exp

@@ -0,0 +1,41 @@
+# Copyright 2019 Free Software Foundation, Inc.
+
+# This program is free software; you can redistribute it and/or modify
+# it under the terms of the GNU General Public License as published by
+# the Free Software Foundation; either version 3 of the License, or
+# (at your option) any later version.
+#
+# This program is distributed in the hope that it will be useful,
+# but WITHOUT ANY WARRANTY; without even the implied warranty of
+# MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+# GNU General Public License for more details.
+#
+# You should have received a copy of the GNU General Public License
+# along with this program.  If not, see <http://www.gnu.org/licenses/>.
+
+standard_testfile exceptprint.cc
+
+if {[skip_cplus_tests]} {
+    return -1
+}
+
+if {[prepare_for_testing "failed to prepare" $testfile $srcfile {debug c++}]} {
+    return -1
+}
+
+if {![runto_main]} {
+    return -1
+}
+
+if { [skip_libstdcxx_probe_tests] == 0 } {
+    untested "Have libstdc++ stap probe"
+    return -1
+}
+
+gdb_test "catch throw" "Catchpoint $decimal \\(throw\\)" \
+    "catch throw"
+
+gdb_test "continue" "Continuing.*Catchpoint $decimal.*"
+
+gdb_test "print \$_exception" \
+    "did not find exception probe \\(does libstdcxx have SDT probes\\?\\).*"