Add a custom error message in EG WaitUtil

This CL adds a parameter to the WaitUntilCondition of Earl Grey wait_util to
display a more explicit custom error message if the condition is not met at
the end of the timeout.

BUG=none

Review-Url: https://codereview.chromium.org/2252723008
Cr-Commit-Position: refs/heads/master@{#413416}
2 files changed