Repair Executor error handling for other classes of IOError/OSError.

[kwlzn]

Addresses handling of @benley's case of attempted execution of a dir, which produced the following unhandled AttributeError:

Traceback (most recent call last):
  File ".bootstrap/_pex/pex.py", line 328, in execute
  File ".bootstrap/_pex/pex.py", line 260, in _wrap_coverage
  File ".bootstrap/_pex/pex.py", line 292, in _wrap_profiling
  File ".bootstrap/_pex/pex.py", line 371, in _execute
  File ".bootstrap/_pex/pex.py", line 429, in execute_entry
  File ".bootstrap/_pex/pex.py", line 434, in execute_module
  File "/usr/lib/python2.7/runpy.py", line 180, in run_module
    fname, loader, pkg_name)
  File "/usr/lib/python2.7/runpy.py", line 72, in _run_code
    exec code in run_globals
  File "/nonexistent/.pex/install/pex_wrapper-0.0.0-py2.7.egg.355ab1e4a2649222db04cd7b5a7f2a400079ce2a/pex_wrapper-0.0.0-py2.7.egg/pex_wrapper.py", line 256, in <module>
    sys.exit(main())
  File "/nonexistent/.pex/install/pex_wrapper-0.0.0-py2.7.egg.355ab1e4a2649222db04cd7b5a7f2a400079ce2a/pex_wrapper-0.0.0-py2.7.egg/pex_wrapper.py", line 253, in main
    pex_builder.build(output)
  File "/nonexistent/.pex/install/pex-1.1.13-py2.py3-none-any.whl.54e2486bd5c6fda99021a6500098c4976315d6fc/pex-1.1.13-py2.py3-none-any.whl/pex/pex_builder.py", line 424, in build
    self.freeze(bytecode_compile=bytecode_compile)
  File "/nonexistent/.pex/install/pex-1.1.13-py2.py3-none-any.whl.54e2486bd5c6fda99021a6500098c4976315d6fc/pex-1.1.13-py2.py3-none-any.whl/pex/pex_builder.py", line 411, in freeze
    self._precompile_source()
  File "/nonexistent/.pex/install/pex-1.1.13-py2.py3-none-any.whl.54e2486bd5c6fda99021a6500098c4976315d6fc/pex-1.1.13-py2.py3-none-any.whl/pex/pex_builder.py", line 335, in _precompile_source
    compiled_relpaths = compiler.compile(self._chroot.path(), source_relpaths)
  File "/nonexistent/.pex/install/pex-1.1.13-py2.py3-none-any.whl.54e2486bd5c6fda99021a6500098c4976315d6fc/pex-1.1.13-py2.py3-none-any.whl/pex/compiler.py", line 86, in compile
    out, _ = Executor.execute([self._interpreter.binary, fp.name])
  File "/nonexistent/.pex/install/pex-1.1.13-py2.py3-none-any.whl.54e2486bd5c6fda99021a6500098c4976315d6fc/pex-1.1.13-py2.py3-none-any.whl/pex/executor.py", line 83, in execute
    stdout_raw, stderr_raw = process.communicate(input=stdin_payload)
AttributeError: 'NoneType' object has no attribute 'communicate'

[peiyuwang]

lgtm!

[kwlzn]

thanks @peiyuwang!