Add error messages / custom fastify errors to blob server plugin

[sethvincent]

As noted here there's not currently useful error messages returned from the blob-server endpoints.

It would be good to either:

• add custom errors with error messages for usage in fastify
• ensure that all reply usage includes error messages

[achou11]

just some helpful context from Gregor in the case of serving blobs specifically:

Just to point out that since this server is serving images, error messages do not make a lot of sense, because we are not processing the response in JS code (the response is handled on the platform specific image component).

still think this is worth exploring otherwise

[gmaclennan]

Marking as post-mvp because I don't think this is necessary for MVP