[gdb/symtab] Handle failure to open .gnu_debugaltlink file

[gdb/symtab] Handle failure to open .gnu_debugaltlink file

If we instrument cc-with-tweaks.sh to remove the .gnu_debugaltlink file after
dwz has created it, with test-case
gdb.threads/access-mem-running-thread-exit.exp and target board cc-with-dwz-m
we run into:
...
(gdb) file access-mem-running-thread-exit^M
Reading symbols from access-mem-running-thread-exit...^M
could not find '.gnu_debugaltlink' file for access-mem-running-thread-exit^M
...
followed a bit later by:
...
(gdb) file access-mem-running-thread-exit^M
Reading symbols from access-mem-running-thread-exit...^M
gdb/dwarf2/read.c:7284: internal-error: create_all_units: \
  Assertion `per_objfile->per_bfd->all_units.empty ()' failed.^M
...

The problem is that create_units does not catch the error thrown by
dwarf2_get_dwz_file.

Fix this by catching the error and performing the necessary cleanup, getting
the same result for the first and second file command.

PR symtab/29805
Bug: https://sourceware.org/bugzilla/show_bug.cgi?id=29805