Non-syntax errors are always counted to determine if to exit
early, but then deduplication eliminates them. This can lead
to situations which report "too many errors" and only one
error is shown.

De-duplicate non-syntax errors early, at least the ones that
appear consecutively, and only count the ones actually being
shown. This doesn't work perfectly as they may not appear in
sequence, but it's cheap and good enough.

Fixes #14136.

Change-Id: I7b11ebb2e1e082f0d604b88e544fe5ba967af1d7
Reviewed-on: https://go-review.googlesource.com/23259Reviewed-by: Matthew Dempsky <mdempsky@google.com>