Improve TypeMismatch errors when match exhaustiveness fails

A first step in that direction is to add context to `TypeMismatch` error sand write some more useful error messages when match exhaustiveness fails.

Start by locating the TypeMismatch error definition and the code handling match exhaustiveness failures. Read how these errors are currently constructed, then define completion as adding context to TypeMismatch and producing more useful messages for failed exhaustiveness checks.

Written by the indexing model from the issue text.