Rationale: this code path is primarily executed from the server. A
potential error, say from a corrupt, unsupported or otherwise invalid
user profile should not cause the server to panic (and crash).
It is possible for `computePasswordHash` to return an error, not just an
impossibility.
Test refactoring:
- use 'require' in user profile tests;
- move test case to TestBadPasswordHashVersion;
- update comments in test.
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