Only invoke completed method on unknown arguments if the failure remedy is FAIL

[jamesnetherton]

No description provided.

[jamesnetherton]

Converted to draft as there's something I need to check before merging.

[jamesnetherton]

@ppalaga Do you mind taking a second look at this. I made some tweaks:

• I realised that if on-failure is warn, then you need to process all valid arguments, instead of stopping on the first invalid argument, so I fixed that.

• To make the warning message output a bit friendlier, I thought it'd be best to truncate long arguments (E.g like a classpath) and abbreviate with '...'.

[jamesnetherton]

The test failure is not related - hence merging this.