Update argument error message when matching with <>

[sabiwara]

The error message for var <> "suffix" = hasn't been updated following the support of pinned variables on the left-handside operand.

** (ArgumentError) the left argument of <> operator inside a match should always be a literal binary because its size can't be verified. Got: var

Updated message:

** (ArgumentError) the left argument of <> operator inside a match should always be either a literal binary or an existing variable matched with the pin operator (such as ^some_var) because its size can't be verified. Got: var

Based the phrasing on the similar error following %{var => 1} = 1

error: cannot use variable var as map key inside a pattern. Map keys in patterns can only be literals (such as atoms, strings, tuples, and the like) or an existing variable matched with the pin operator (such as ^some_var)

[josevalim]

I suggested improvements, then please ship it (and backport!).

[sabiwara]

Sorry I merged too fast, the suggestion broke the test.
Will backport together with #13327.

[sabiwara]

Thanks! Backported