Move cluster verify error codes to constants

- move the cluster verify error codes from cmdlib._VerifyErrors to
  constants;
- add to each of them the CV (Cluster Verify) prefix;
- add the CV_ALL_ECODES and CV_ALL_ECODES_STRINGS constants;
- wrap the lines that exceed 80 characters after changing the error
  code names to the new ones.

Signed-off-by: Andrea Spadaccini <spadaccio@google.com>
Reviewed-by: Michael Hanselmann <hansmi@google.com>