fix: clarify external error messages

[ghassan-gaidi]

Improve two error messages to help users identify the root cause faster:

Changes

src/conversation.ts - wait() error message now explicitly mentions that Bot API calls must not be made inside external():

"Cannot wait for updates from inside external, or concurrently to it! Bot API calls (e.g. ctx.api.*, ctx.reply) must not be made inside external. First return your data from external and then resume update handling using wait calls."

src/state.ts - Bad replay error now shows both the expected and actual operation, plus a hint about the common cause:

"Bad replay, expected op '...' but got '...'. This usually means a Bot API call was made inside `conversation.external()` or the conversation was otherwise used incorrectly."

Closes #148

[KnorpelSenf]

Did you try to reproduce these issues before you adjusted the error messages? I don't think that they're both caused by the same issue, so this seems weirdly off. Was this maybe fully AI-generated?