Don't warn for an unreferenced exception symbol in a catch block.

A catch block may not use the exception variable, in which case UJS will warn about it being unreferenced. Since there is no way to write a catch block in JS without specifying a variable to hold the exception, this warning is superfluous.

This change checks that a symbol is not an AST_SymbolCatch before warning about it being unreferenced.

Before this change:

```sh
$ echo 'try { } catch (ex) { }' | ./uglifyjs --lint

WARN: Symbol ex is declared but not referenced [-:1,15]
try{}catch(ex){}
```

After this change:

```sh
$ echo 'try { } catch (ex) { }' | ./uglifyjs --lint

try{}catch(ex){}
```