[avalanche] Capitalize the init error messages and add a dot

Summary:
This makes the error messages more consistent with others from our
codebase.

Ref T1635.

Test Plan:

ninja check-functional

Reviewers: #bitcoin_abc, majcosta

Reviewed By: #bitcoin_abc, majcosta

Maniphest Tasks: T1635

Differential Revision: https://reviews.bitcoinabc.org/D9667