The test "assertThrows(builder.instantiate)" threw a TypeError before,
which made the test pass, but not because of the feature we wanted to
test.
This CL fixes the test to call builder.instantiate correctly, and also
tests for the correct error message.

Drive-by fix: Fix {expected} and {found} parameters in assertThrows.

R=ahaas@chromium.org

Change-Id: I11c0f63885cc14a36559e637aea60a9da6f1bb8f
Reviewed-on: https://chromium-review.googlesource.com/472886Reviewed-by: Andreas Haas <ahaas@chromium.org>
Commit-Queue: Clemens Hammacher <clemensh@chromium.org>
Cr-Commit-Position: refs/heads/master@{#44584}