Raise `re2.error` instead of crashing.

Fixes #484.

Change-Id: I152b5ed8a6358d2d74f553bde2e66f2e50cfba1d
Reviewed-on: https://code-review.googlesource.com/c/re2/+/62890
Reviewed-by: Alex Chernyakhovsky <achernya@google.com>
Reviewed-by: Paul Wankadia <junyer@google.com>