From time to time, often due to hand-manipulation of data or unfortunate

crashes, a stored database signature may not contain some data expected

by evolution files, resulting in KeyErrors when attempting to apply the

evolution. It's not always clear what part of the database or evolution

process these KeyErrors apply to, making these situations painful for

debugging.
This change adds better error checking around any signature lookups,

raising errors with enough information to identify the change being

applied. It also improves existing error messages, creating consistent

wording for most mutation-related errors.

Unit tests pass.