Since ValidateExpression just throws the only queued (first) expression error,
we can delay throwing it until the latest possible moment. That's right before
the matching expression classifier dies (goes out of scope or accumulates).

Change-Id: I4538de333b789ae786278b94b76b4799ccdf4903
Reviewed-on: https://chromium-review.googlesource.com/c/1286678
Commit-Queue: Toon Verwaest <verwaest@chromium.org>
Reviewed-by: Adam Klein <adamk@chromium.org>
Cr-Commit-Position: refs/heads/master@{#56785}