Introduce parseable error codes in LUVerifyCluster

Currently the output of cluster verify can be parsed for 'ERROR'
messages, but that is the only indication we get (error or no error). In
order to allow monitoring tools to separate different error conditions,
this patch introduces a new output format (“gnt-cluster verify
--error-codes”) that changes the output from human-friendly to
machine-friendly. In this mode, an error line changes from:
  ERROR: node node1: drbd minor 1 of instance inst1.is not active

to:
  ERROR:ENODEDRBD:node:node1:drbd minor 1 of instance inst1 is not active

i.e. the error message is a ‘:’-separated field, with ERROR in the first
place, the error code in the second, the object type (cluster, node,
instance) in the third, the name of the object (for nodes/instances) in
the fourth, and then the text message.

The patch also removes some of the verbosity of the operation
(“Verifying instance X”, “Verifying node X”) since on big clusters these
informational messages can quickly fill up an entire screen. The
original behaviour can be restored via the ‘--verbose’ option.

Signed-off-by: Iustin Pop <iustin@google.com>
Reviewed-by: Michael Hanselmann <hansmi@google.com>