[btcsuite/btcec] d4d2f6: Fix bug and inconsistant error msg seen by lint.

[John C. Vernaleo]

  Branch: refs/heads/master
  Home:   https://github.com/btcsuite/btcec
  Commit: d4d2f622b567a05cedfa7e6d05ec4fb71a2f5d37
      
https://github.com/btcsuite/btcec/commit/d4d2f622b567a05cedfa7e6d05ec4fb71a2f5d37
  Author: John C. Vernaleo <j...@conformal.com>
  Date:   2015-02-03 (Tue, 03 Feb 2015)

  Changed paths:
    M btcec.go
    M signature.go

  Log Message:
  -----------
  Fix bug and inconsistant error msg seen by lint.