Tools: improved error handling for unitc

[lcrilly]

This patch does a number of things to help when failing to apply a new configuration.

• The error body from the Unit control API is displayed which can have useful troubleshooting information (when the version of curl supports it).

• When using the EDIT option, the temporary file with unapplied changes is preserved so that the user can edit it again without losing their work.

• Editing JavaScript modules no longer requires that module to have been enabled.

• Failure to apply edited JavaScript modules now rolls-back to the previous configuration instead of deleting the module.

[callahad]

Looks reasonable to me. @ac000 can you handle merging please

[ac000]

Rebased with master...