This forces all return values to be checked for gl::Error.
Requires quite a bit of minor refactoring. I also added a macro to
swallow an error without returning from a function.

We could look at storing the errors in the Context at some point,
since almost always when we're generating errors that we need to
discard we have access to the Context as a parameter.

BUG=angleproject:2150

Change-Id: I457e48a30c002eda0993acbcd3180ba87bf169fb
Reviewed-on: https://chromium-review.googlesource.com/665173
Commit-Queue: Jamie Madill <jmadill@chromium.org>
Reviewed-by: Yuly Novikov <ynovikov@chromium.org>