Improve error messages for assertToken

[juliaogris]

assertToken, assertEOL have confusing error messages.
Rework globally.

Consider adding more structured error messages (markdown? fix-suggestion?).

print ("hello" "world")

error:

expected ")", got ""
expected end of line, found "world"

This should be,

1. A single error, not two.
2. Read

   invalid `"world"`, did you mean `)` or operator?