Apple Clang 8.1 doesn't allow implicit conversion from gl::Error to
egl::Error with the egl::Error::Error(gl::Error&&) constructor. Same
thing for gl and egl reversed. This commits add conversion constructors
taking errors by value. There should be not performance impact for
non-error code paths.

BUG=

Change-Id: I91acf094af923080780b91850146d71016ec5ebc
Reviewed-on: https://chromium-review.googlesource.com/671284Reviewed-by: Corentin Wallez <cwallez@chromium.org>
Reviewed-by: Jamie Madill <jmadill@chromium.org>
Commit-Queue: Corentin Wallez <cwallez@chromium.org>