Replace unreachable("templateTypeArgs.length != sourceTypeArgs.length") with a real error

[kengorab]

When attempting to extract generics from a "template" type during typechecking, if the number of type arguments does not match the number of type arguments in the "source" type, we reach an unreachable. However, this is a completely valid error state to be in and should be raised as a normal TypeError rather than crashing the program.