highlight the error condition when reporting a config issue
highlight the error condition when reporting a config issue

Add the word "ERROR" to make it easier to find the error when searching
the loooong output produced by zuul-server when checking its config
file, to make debugging failures of the validation tests in patches to
project-config simpler.

Change-Id: I38cf31619eb309ebb7fc49eca4437a9c36bcb1ce
Signed-off-by: Doug Hellmann <0798ebecfb7bc4dfb9d0445c2fa030eb64e1fa1c@doughellmann.com>

gooddata/zuul,gooddata/zuul,wikimedia/integration-zuul,gooddata/zuul,gooddata/zuul,wikimedia/integration-zuul,gooddata/zuul,wikimedia/integration-zuul,wikimedia/integration-zuul