Get rid of the last call to AddErrorContext, and its definition.

There are still some spurious 'if not w: return None' statements.  But
I believe all the 'fundamental' errors use exceptions now.

osh/word_parse.py

@@ -112,21 +112,9 @@ def Reset(self, lex_mode=lex_mode_e.OUTER):

    self.error_stack = []

  def AddErrorContext(self, msg, *args, **kwargs):
    err = util.ParseError(msg, *args, **kwargs)
    self.error_stack.append(err)

  def Error(self):
    return self.error_stack

  def _BadToken(self, msg, token):
    """
      Args:
        msg: format string with a single %s token
        token: Token
    """
    self.AddErrorContext(msg, token, token=token)

  def PrevToken(self):
    """Inspect state.  Used by completion.

@@ -296,7 +284,7 @@ def _ParseVarOf(self):
    self._Peek()  # Check for []
    if self.token_type == Id.VOp2_LBracket:
      bracket_op = self._ReadSubscript()
      if not bracket_op: return None
      assert bracket_op is not None
    else:
      bracket_op = None

@@ -309,7 +297,7 @@ def _ParseVarExpr(self, arg_lex_mode):
    Start parsing at the op -- we already skipped past the name.
    """
    part = self._ParseVarOf()
    if not part: return None
    assert part is not None

    self._Peek()
    if self.token_type == Id.Right_VarSub:
@@ -322,36 +310,27 @@ def _ParseVarExpr(self, arg_lex_mode):
    if op_kind == Kind.VTest:
      op_id = self.token_type
      arg_word = self._ReadVarOpArg(arg_lex_mode)
      if self.token_type != Id.Right_VarSub:
        # NOTE: Not sure how to tickle this.  May not be possible.
        self._BadToken('Unexpected token after test arg: %s', self.cur_token)
        return None
      assert self.token_type == Id.Right_VarSub, self.cur_token

      part.suffix_op = ast.StringUnary(op_id, arg_word)

    elif op_kind == Kind.VOp1:
      op_id = self.token_type
      arg_word = self._ReadVarOpArg(arg_lex_mode)
      if self.token_type != Id.Right_VarSub:
        # NOTE: Not sure how to tickle this.  May not be possible.
        self._BadToken('Unexpected token after unary op: %s', self.cur_token)
        return None
      assert self.token_type == Id.Right_VarSub, self.cur_token

      op = ast.StringUnary(op_id, arg_word)
      part.suffix_op = op
      part.suffix_op = ast.StringUnary(op_id, arg_word)

    elif op_kind == Kind.VOp2:
      if self.token_type == Id.VOp2_Slash:
        op = self._ReadPatSubVarOp(arg_lex_mode)
        if not op:
          return None
        assert op is not None
        # Checked by the method above
        assert self.token_type == Id.Right_VarSub, self.cur_token

      elif self.token_type == Id.VOp2_Colon:
        op = self._ReadSliceVarOp()
        if not op:
          return None
        assert op is not None
        # NOTE: } in arithmetic mode.
        if self.token_type != Id.Arith_RBrace:
          # Token seems off; doesn't point to X in # ${a:1:2 X