refactor: use existing IllegalVariableInPatternAlternative message

[ckipp01]

We have a IllegalVariableInPatternAlternative message that already
exists and was used in Desugar, but not in the more common place
people will see it in Typer. This just changes the message to align
with the one that people are used to seeing, but also adds in the
explain since we're re-using the existing message.

Example before

scala> Option((1, 2)) match
     |   case (1, n) | (n, 1) => "got a one!"
     |
-- Error: ----------------------------------------------------------------------
2 |  case (1, n) | (n, 1) => "got a one!"
  |           ^
  |           Illegal variable n in pattern alternative
-- Error: ----------------------------------------------------------------------
2 |  case (1, n) | (n, 1) => "got a one!"
  |                 ^
  |                 Illegal variable n in pattern alternative
2 errors found

Example after

scala> Option((1, 2)) match
     |   case (1, n) | (n, 1) => "got a one!"
     |
-- [E024] Syntax Error: --------------------------------------------------------
2 |  case (1, n) | (n, 1) => "got a one!"
  |           ^
  |           Illegal variable n in pattern alternative
  |
  | longer explanation available when compiling with `-explain`
-- [E024] Syntax Error: --------------------------------------------------------
2 |  case (1, n) | (n, 1) => "got a one!"
  |                 ^
  |                 Illegal variable n in pattern alternative
  |
  | longer explanation available when compiling with `-explain`
2 errors found