Clarify the error message for an ECDSA power-on test failure.
We want to clarify that this isn't the PWCT that FIPS generally means,
but rather the power-on self-test. Since ECDSA is non-deterministic, we
have to implement that power-on self-test as a PWCT, but we have a
different flag to break that actual PWCT.
Change-Id: I3e27c6a6b0483a6c04e764d6af8a4a863e0b8b77
Reviewed-on: https://boringssl-review.googlesource.com/16765
Reviewed-by: Adam Langley <agl@google.com>