Don't crash cover on insufficient_vnodes_available

* Change `calc_plan` to return error-tuple rather than match it and
  throw it.

* Check for error-tuple when generating cached coverage plans and
  store undefined.  That will prevent cover server from crashing.

* Check for error-tuple during search request and return proper error
  msg to user.
Merge branch 'master' into rz-dont-crash-cover
Use insufficient_vnodes_available return value for PB
Merge pull request #115 from basho/rz-dont-crash-cover

Don't crash cover on insufficient_vnodes_available