Replace panics with error messages

There are a lot of places in Dolt and GMS where we intentionally panic when encountering unexpected values. This makes for a bad user experience. Instead, we should error out with a message.

Start by surveying the panic sites across Dolt and GMS, since the issue does not identify particular files, tests, or entry points. Replace intentional panics for unexpected values with user-facing error messages, and consider the work done when those paths error out instead of panicking.

Written by the indexing model from the issue text.