Throw more actionable "Conflict" exceptions?

[vemv]

Hi James,

while using ragtime I experienced the following error message:

Conflict! Expected 20210722233700-add-foo but 20210727114336-add-bar was applied.

...while ultimately I was able to solve the underlying cause, I found it a bit frustrating how concise the exception was.

One could say, a great exception message of the "precondition" kind explains why a precondition was expected, and a suggested action or diagnostic.

In particular:

• why was certain migration "expected"? what controls ragtime's expectations?
  • i.e., I have no idea what's wrong here - file naming, some internal piece of state, the DB's state etc
• what can I do to fix this?
  • perhaps a detailed diagnostic for the precise "red path" would be too much of an ask. But a generalized explanation or maybe a link to a specific passage of the documentation could help.

Cheers - V