Dotty crashes while typechecking invalid super prefix

[abeln]

When typechecking

class A { C.super.foo() }

the typer throws with the following stacktrace: http://pastebin.com/arjFaB0Z
The issue is an unguarded pattern matching in TypeAssigner: https://github.com/lampepfl/dotty/blob/d0621108bad55f9fc66c1c5ade9a0b7edb3117e7/compiler/src/dotty/tools/dotc/typer/TypeAssigner.scala#L294

The fix (I think) is to simply handle the case where the qualifier has type ErrorType and bail out early.

[abeln]

Sent #2118 for this.