Error recovery and poison types

[ramonasuncion]

Continue past errors and collect multiple diagnostics instead of stopping at the first one.

var x : Int = "oops";   // error: type mismatch
var y = x + 1;          // suppressed: x already failed

• Dedup collected errors so the same error at the same location only appears once.
• Cap output with a "too many errors" note when the count gets too high.