Display error in catch(...)

A catch(...) was hiding a problem we had - it would have been faster to debug if it had printed a message, this PR makes it do so.

Author: NathanJPhillips

src/goto-analyzer/taint_analysis.cpp

@@ -400,6 +400,7 @@ bool taint_analysist::operator()(
   }
   catch(...)
   {
+    error() << "Caught unexpected error in taint_analysist::operator()" << eom;
     return true;
   }
 }