better output for confrontation objects

[markvanderloo]

More user-friendly print method. e.g. also showing the number of missings.

[markvanderloo]

New output; comments welcome.

> check_that(SBS2000, turnover >= 0, other.rev >= 0, foo >= 1 )
Object of class 'validation'
Call:
    check_that(SBS2000, turnover >= 0, other.rev >= 0, foo >= 1)

Rules confronted: 3
   With fails   : 1
   With missings: 2
   Threw warning: 0
   Threw error  : 1

[matthiasgomolka]

Showing the number of rules with missings is really helpful!

[edwindj]

This is surely an improvement!

I opt for an extra statement whenever the number of missings is > 0.
Something along the lines:

Note: in some cases when rules interact, missings may indicate an invalidation.