This makes the log events easier to parse, and makes it easier to
consistently use the correct fields from the issuance request.
Also, reduce the number of fields that are logged on error events.
Logging just the serial and the error in most cases should suffice to
cross-reference the error with the item that we attempted to sign.
One downside is that this increases the total log size (10kB above, vs
7kB from a similar production issuance) due in part to more repetition.
For example, both the "signing cert" and "signing cert success" log
lines include the full precert DER.
Note that our long-term plan for more structured logs is to have a
unique event id to join logs on, which can avoid this repetition. But
since we don't currently have convenient ways to do that join, some
duplication (as we currently have in the logs) seems reasonable.
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