Better error messages

Cool error messages like this:

$ necroparse file.sk
File "test.sk", line 3, characters 14-15:
   3 | val f<a> (x: }
                    ^
Invalid character
$

$ necroparse file.sk
File "test.sk", line 3, characters 17-18:
   3 | val f<a> (x: a) =
                       ^
Syntax error, unexpected token: =
$

$ necroparse file.sk
File "test.sk", line 3, characters 11-15:
   3 | val f<a> (type
                 ^^^^
Syntax error, unexpected token: type
$