Fix exception catch in buck.py etc for Python 3.5 sytanx

Created by: anorth

Python 3.5 no longer accepts the comma style of naming an exception.

With .buckconfig interpreter set to python3.5:

~/projects/mt/redacted[alex/buck]$ buck build redacted:server
Warning raised by BUCK file parser:   File "/Users/alex/projects/mt/redacted/.buckd/tmp/buck_run.r7CpLP/buck3671387140942854562.py", line 200
Warning raised by BUCK file parser:     except build_env.watchman_error, e:
Warning raised by BUCK file parser:                                    ^
Warning raised by BUCK file parser: SyntaxError: invalid syntax
Parse error for build file /Users/alex/projects/mt/redacted/redacted/BUCK: Parser exited unexpectedly