[gdb] Catch exception when constructing the highlighter

[gdb] Catch exception when constructing the highlighter

Currently in source_cache::ensure we catch the exception that triggers when
highlighter->highlight is called:
...
         try
           {
             std::istringstream input (contents);
             std::ostringstream output;
             highlighter->highlight (input, output, lang_name, fullname);
...
and the file used earlier in the construction of the highlighter:
...
             highlighter = new srchilite::SourceHighlight ("esc.outlang");
...
is missing.

The fact that this exception triggers when highlighter->highlight is called is
an implementation artefact of libsource-highlight.so though, and this could be
different for older or newer versions.

Make things more robust by also catching exceptions thrown during construction
of the highlighter.  This makes the handling on master equivalent with what
has been committed for 8.3.1.

Tested on x86_64-linux.

gdb/ChangeLog:

2019-09-19  Tom de Vries  <tdevries@suse.de>

	PR gdb/25009
	* source-cache.c (source_cache::ensure): Catch exception thrown during
	construction of the highlighter.