avoid dependency on ldap by optionally faking the exceptions

[graingert]

[graingert]

@eeshangarg we currently use python-ldap~=2.4 and although we are looking to upgrade we're not yet ready to do so. When installing this testing package we are forced to upgrade to python-ldap~=3.0 which conflicts

[graingert]

this also allows people to test without having python-ldap installed at all:

import sys
import fakeldap

sys.modules['ldap'] = fakeldap.ldap

[eeshangarg]

@graingert: Thanks for working on this! Looks good to me! :)

@timabbott: Could you please take a quick look as well? Thanks!

[timabbott]

Makes sense to me; posted a quick comment on documentation.

[timabbott]

Possibly we should also add a brief section at the bottom of the README mentioning this behavior, maybe with a title like "Usage without ldap installed".