Bad TLS cert error messages could be more helpful

Low priority because the experience is pretty tolerable. You can figure it out with some effort.

Hitting silo create with a bad request body (cert: 'Hello\n') results in the following error message. But there is no certificate field (it's cert). Also, the body contains an array of certs and this error doesn't tell you which one has the error. It blows up on the first bad one.

Unsupported value for "certificate": Certificate exists, but is empty.

The request looked like this:

Relevant code

https://github.com/oxidecomputer/omicron/blob/47a6b42c986c65292ee61b0c79090fa57dec5fe9/certificates/src/lib.rs#L24-L25

https://github.com/oxidecomputer/omicron/blob/47a6b42c986c65292ee61b0c79090fa57dec5fe9/nexus/db-model/src/certificate.rs#L42-L59

https://github.com/oxidecomputer/omicron/blob/47a6b42c986c65292ee61b0c79090fa57dec5fe9/nexus/db-queries/src/db/datastore/silo.rs#L235-L248

Start with certificates/src/lib.rs, nexus/db-model/src/certificate.rs, and nexus/db-queries/src/db/datastore/silo.rs, especially the linked lines, then reproduce silo creation with the bad cert request body. Done means the error refers to the cert field and identifies which certificate in the array is invalid.