Avoid expression parsing crash with unknown language

PR gdb/28093 points out that gdb crashes when language is set to
"unknown" and expression parsing is attempted.  At first I thought
this was a regression due to the expression rewrite, but it turns out
that older versions crash as well.

This patch avoids the crash by changing the default expression parser
to throw an exception.  I think this is preferable -- the current
behavior of silently doing nothing does not really make sense.

Bug: https://sourceware.org/bugzilla/show_bug.cgi?id=28093

gdb/language.c

@@ -825,8 +825,8 @@ public:
 
   int parser (struct parser_state *ps) const override
   {
-    /* No parsing is done, just claim success.  */
-    return 1;
+    error (_("expression parsing not implemented for language \"%s\""),
+	   natural_name ());
   }
 
   /* See language.h.  */

gdb/testsuite/gdb.base/langs.exp

@@ -111,6 +111,12 @@ if [runto csub] then {
              "set lang to minimal"
     
     gdb_test "print x" " = 5000" "print parameter value"
+
+    # Ensure this at least does not crash.
+    gdb_test_no_output "set language unknown"
+    gdb_test "print x" \
+	"expression parsing not implemented for language .Unknown." \
+	"expression parsing does not crash for unknown"
 }
 
 set timeout $oldtimeout