rustc shouldn't stop after the first error

[pcwalton]

After the first error, we should keep going in some cases. As a first stab at this, we could abandon the current function and try to resolve/typecheck other functions.

[brson]

So there is a mechanism for this now and a lot of the most frequent errors are non-fatal, but I'm unassigning myself because I don't have any plans for 'finishing' this task.

[graydon]

if a mechanism exists and we are actively using it, i am ok closing the bug.