lib: fix non-local lambda expression cannot have a capture-default

We got away with this because GCC 4.8 (and apparently every GCC prior
to 9) didn't notice or care, and because there is nothing referenced
inside the lambda function body that isn't accessible from any other
kind of function body (i.e. the capture wasn't needed at all).

GCC 9 now enforces what the C++ standard said all along:  there is
no need to allow capture-default in this case, so it is not.

Fix by removing the offending capture-default.

Fixes: https://github.com/Zygo/bees/issues/112
Signed-off-by: Zygo Blaxell <bees@furryterror.org>

lib/error.cc

@@ -32,7 +32,7 @@ namespace crucible {
 
 	// FIXME:  could probably avoid some of these levels of indirection
 	static
-	function<void(string s)> current_catch_explainer = [&](string s) {
+	function<void(string s)> current_catch_explainer = [](string s) {
 		cerr << s << endl;
 	};