Fix unhandled exception in 'source' when there's a syntax error.

The spec tests shows that shells disagree on the exit code, and that
dash is more strict.

core/cmd_exec.py

@@ -182,15 +182,12 @@ def _Complete(self, argv):
  def _EvalHelper(self, c_parser, source_name):
    self.arena.PushSource(source_name)
    try:
      node = c_parser.ParseWholeFile()
      # NOTE: We could model a parse error as an exception, like Python, so we
      # get a traceback.  (This won't be applicable for a static module
      # system.)
      if not node:
      try:
        node = c_parser.ParseWholeFile()
      except util.ParseError as e:
        util.error('Parse error in %r:', source_name)
        err = c_parser.Error()
        ui.PrintErrorStack(err, self.arena, sys.stderr)
        return 1
        ui.PrettyPrintError(e, self.arena, sys.stderr)
        return 2

      status = self._Execute(node)
      return status

spec/builtin-eval-source.test.sh

@@ -57,3 +57,14 @@ func
foo=foo_val
foo=
## END

#### Source with syntax error
# TODO: We should probably use dash behavior of a fatal error.
echo 'echo >' > $TMP/syntax-error.sh
. $TMP/syntax-error.sh
echo status=$?
## stdout: status=2
## OK bash/mksh stdout: status=1
## OK zsh stdout: status=126
## OK dash stdout-json: ""
## OK dash status: 2