[gdb/testsuite] Fix gdb.guile/scm-breakpoint.exp with guile 3.0

When running test-case gdb.guile/scm-breakpoint.exp on openSUSE Tumbleweed
with guile 3.0, I run into:
...
(gdb) guile (define cp (make-breakpoint "syscall" #:type BP_CATCHPOINT))^M
ERROR: In procedure make-breakpoint:^M
In procedure gdbscm_make_breakpoint: unsupported breakpoint type in \
  position 3: "BP_CATCHPOINT"^M
Error while executing Scheme code.^M
(gdb) FAIL: gdb.guile/scm-breakpoint.exp: test_catchpoints: \
  create a catchpoint via the api
...

The same test passes on openSUSE Leap 15.2 with guile 2.0, where the second
line of the error message starts with the same prefix as the first:
...
ERROR: In procedure gdbscm_make_breakpoint: unsupported breakpoint type in \
  position 3: "BP_CATCHPOINT"^M
...

I observe the same difference in many other tests, f.i.:
...
(gdb) gu (print (value-add i '()))^M
ERROR: In procedure value-add:^M
In procedure gdbscm_value_add: Wrong type argument in position 2: ()^M
Error while executing Scheme code.^M
(gdb) PASS: gdb.guile/scm-math.exp: catch error in guile type conversion
...
but it doesn't cause FAILs anywhere else.

Fix this by updating the regexp to make the "ERROR: " prefix optional.

Tested on x86_64-linux, with both guile 2.0 and 3.0.

gdb/testsuite/ChangeLog:

2021-07-08  Tom de Vries  <tdevries@suse.de>

	* gdb.guile/scm-breakpoint.exp: Make additional "ERROR: " prefix in
	exception printing optional.