Improve inv-validate output

[PeterJCLaw]

At the moment it puts out a very human-readable string with very little information. While the key information is present, the tool could very easily output much more useful information.

Example (from memory) of the current output (on failure):

Duplicate assets found: 1a2b, 1b2c

Suggestion for new output:

Duplicate assets found:
# sr1a2b
* dave/home/keyboard-cat-sr1A2B
* oxford/kit-checkin-location/keyboard-cat-sr1A2B
Asset files are identical

# sr1b2c
* dave/home/mouse-sr1B2C
* oxford/kit-checkin-location/mouse-sr1B2C
Asset files differ:
 assetcode : 1B2C
 labelled  : true   # Does the asset have its assetcode marked on it
-revision  : 0
+revision  : 1

 description: >-

This output contains much more diagnostic information to aid in resolving the validation error as well as being greppable (^# sr(\w+)$) for part codes if the user wants to pass them to another command.