Add custom code for CV_E* constants to convert-constants

Currently, the cluster verify errors are defined as follows:

CV_ECLUSTER_FOO = (TCLUSTER, "ECLUSTER_FOO", "description")

This means there's no standalone name for the string "ECLUSTER_FOO",
which makes it hard to derive automatically a type for this union in
Haskell.

There are three possible fixes:

- manually separate the CV_ECLUSTER_FOO_STR = "ECLUSTER_FOO" in
  constants.py
- manually extract the strings in Haskell code
- change convert-constants to automatically export virtual constants
  for those

After discussion on IRC, I've taken the latter approach; even though a
bit hack-ish, it avoids manual work and potential errors.

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