Merge pull request #43 from teknico/vp

Lowercased error messages, cleared up comments, shortened some lines.
Add test of decryptExtendedKey error paths
Add code review corrections
Merge pull request #45 from larshesel/vp

Add test of decryptExtendedKey error paths
pool_test.go reformatting
- uniformed all messages with actual value first, expected second
- clarified error messages
- made comments more readable
- shortened long lines

No functional changes.