[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