check-code: detect legacy exception syntax

We just rewrote all files to use modern exception syntax. Ban the old
form.

This will detect the "except type, instance" and
"except (type1, type2), instance" forms.

contrib/check-code.py

@@ -281,6 +281,8 b' pypats = ['
     (r'\.debug\(\_', "don't mark debug messages for translation"),
     (r'\.strip\(\)\.split\(\)', "no need to strip before splitting"),
     (r'^\s*except\s*:', "naked except clause", r'#.*re-raises'),
+    (r'^\s*except\s([^\(,]+|\([^\)]+\))\s*,',
+     'legacy exception syntax; use "as" instead of ","'),
     (r':\n(    )*( ){1,3}[^ ]', "must indent 4 spaces"),
     (r'ui\.(status|progress|write|note|warn)\([\'\"]x',
      "missing _() in ui message (use () to hide false-positives)"),