Description of problem: When trying to add a custom field to the usersearch
using config-mod does not specify the error. For example, i was trying to add a
custom field and it said "not allowed" and "Validation Error", after some debug
i matched the custom field must be in lowercase.

Version-Release number of selected component (if applicable): Freeipa 4.2.3 and
4.2.4


How reproducible: Trying adding some custom field with wrong sintax.


Steps to Reproduce:
1. ipa -d config-mod --usersearch="uid,givenname,sn,cn,mail,myCustomField"
fail

2. ipa -d config-mod --usersearch="uid,givenname,sn,cn,mail,mycustomField"
ok

Actual results:
ipa: DEBUG: stderr=
ipa: DEBUG: Starting external process
ipa: DEBUG: args='keyctl' 'pupdate' '340300374'
ipa: DEBUG: Process finished, return code=0
ipa: DEBUG: stdout=
ipa: DEBUG: stderr=
ipa: DEBUG: Destroyed connection context.rpcclient_140074165811664
ipa: ERROR: invalid 'ipausersearchfields': attribute "myCustomField" not
allowed

Expected results: ok (config changed)

Additional info: For the record i added the custom schema 1 month ago, and
after that i restarted the ipa server several times, and i already have some
users with that custom field.

Opening this bug as requested by rcrit.