Show exception message from filter_program_output.

As opposed to dumping a Perl object straight to the terminal.

kdesrc-build

@@ -3912,6 +3912,10 @@ EOF
 
         if($@)
         {
+            if (ref $@ && $@->isa('BuildException')) {
+                $@ = $@->{message};
+            }
+
             error ("Unable to run r[b[svn], is the Subversion program installed?");
             error (" -- Error was: r[$@]");
             return undef;