Multiple error message for wrong assignments

[sharkdp]

We used to just have a single descriptive error message when attempting to do an assignment like x = 3:

>>> x = 3
error: while parsing
  ┌─ <input:1>:1:3
  │
1 │ x = 3
  │   ^ Trailing '=' sign. Use `let x = …` if you intended to define a new constant.

but now we get

>>> x = 3
error: while parsing
  ┌─ <input:1>:1:3
  │
1 │ x = 3
  │   ^ Trailing '=' sign. Use `let x = …` if you intended to define a new constant.

error: while parsing
  ┌─ <input:1>:1:3
  │
1 │ x = 3
  │   ^ Expected one of: number, identifier, parenthesized expression

presumably since #228 (FYI @irevoire)

[irevoire]

I'll look into it this week!