dire-warning detour if invalid BIP-85 child index

[jdlcdl]

resolves issue #369

This pr adds a detour to Dire Warning whenever a BIP-85 Child Index is outside the valid range of (0, 2**31-1), informing the user and forcing them to "Try Again", instead of the application crashing.

Related to this pr is #370 (handling exceptions which have no exception message), but they are independent of each other.

• if this one is merged but not the other one: the bug reported in BIP85 child seed - index variable overflow #369 will be resolved, and the exception handler bug will be disguised until another message-less exception is found.
• if the other one is merged but not this one: users would see a vague "Assertion Error" with embit references, and can start over... guessing what they did wrong... but at least the application won't crash.
• if both are merged, users are informed and can try again AND we might learn of exception-type/filename/line-number the next time seedsigner "hangs" for message-less exceptions.

[newtonick]

ACK Tested

[newtonick]

LGTM