Clarify GN nonfatal unused build arg messages.

Changes the messaging for nonfatal "unused build arg" messages to be
"WARNING" instead of "ERROR" when they are nonfatal and clarify that
the build has continued. Previously it would print "ERROR" and
continue anyway which was confusing.

Add an Err object move constructor and de-inline the assignment
operator (previously implicit).

Bug: 500696
Change-Id: Id6056b7a4d33466485f9e138a890e211f447817f
Reviewed-on: https://chromium-review.googlesource.com/762086
Commit-Queue: Brett Wilson <brettw@chromium.org>
Reviewed-by: Will Harris <wfh@chromium.org>
Reviewed-by: Dirk Pranke <dpranke@chromium.org>
Cr-Commit-Position: refs/heads/master@{#516320}
4 files changed