This patch fixes the logic of finding the location of a duplicate
parameter error in arrow functions by only looking at the error if it
exists. This should address a TSAN error. Further, an UNREACHABLE()
statement is inserted somewhere in the ExpressionClassifier code
to make future similar bugs easier to find.

BUG=v8:4483

Review-Url: https://codereview.chromium.org/2365693004
Cr-Commit-Position: refs/heads/master@{#39701}