Update libcephfs.h

Defined error codes for the mount function to get rid of magic numbers.

Update libcephfs.cc

Used error codes from libcephfs.h to replace the magic numbers in the mount functon.

Update libcephfs.cc

Explained where CEPHFS_ERRO_MON_MAP_BUILD, CEPHFS_ERROR_NEW_CLIENT, and CEPHFS_ERROR_MESSENGER_START were defined.