Currently when you use in CLI some command working with LIST or OBJECT and you does not specify that it should be LIST {[]} or OBJECT {} you will general exception which talks about nothing:

"failure-description" => "JBAS014749: Operation handler failed: null",

Please, improve the message for user would know understand that he used incorrect value type.

E.g. 
/subsystem=datasources/data-source=ExampleDS:write-attribute(name=valid-connection-checker-properties, value=1)

should inform the user that incorrect data type was used and OBJECT is needed to be used.

The better message should be used when user uses incorrect syntax, like:
/subsystem=datasources/data-source=ExampleDS:write-attribute(name=valid-connection-checker-properties, value={abc:abc})
instead of
/subsystem=datasources/data-source=ExampleDS:write-attribute(name=valid-connection-checker-properties, value={abc=abc})

The user should get information that incorrect syntax was used and not just info that there is some NullPointerException.

There is quite informative message when you use LIST instead of OBJECT
/subsystem=datasources/data-source=ExampleDS:write-attribute(name=valid-connection-checker-properties, value={[a=a]})
you could see
"failure-description" => "JBAS014688: Wrong type for valid-connection-checker-properties. Expected [OBJECT] but was LIST"

Just I would have a proposal to use text which lead to way how to use OBJECT and LIST. I mean use something like:
"JBAS014688: Wrong type for valid-connection-checker-properties. Expected {OBJECT} but was {[LIST]}"