Improved Output For Errors

[brodieG]

Doesn't make sense to show both value diff and condition diff when in one instance there is an error and in the other there isn't:

> library(ggplot2)
Error in library(ggplot2) : there is no package called ‘ggplot2’
unitizer test fails on value, and conditions mismatches:
*value* mismatch: `.REF` value was not recorded, but `.NEW` value was; they are 
likely different
< .ref                   > .new                 
@@ 1 @@                  @@ 1 @@                
< <object not recorded>  > NULL                 
*conditions* mismatch: condition count mismatch; expected 0 (got 1)
< .REF$conditions                       > .NEW$conditions                     
@@ 1 @@                                 @@ 1,6 @@                             
< Empty condition list                  > Condition list with 1 condition:    
~                                       >   <simpleError in library(ggplot2): 
~                                       >   there is no package called ‘      
~                                       >   ggplot2’>                         
~                                       > You can access conditions directly  
~                                       > (e.g. `.NEW$conditions[[1L]]`).     
Additionally, there are state differences (compare with `diff_state()`).

Also, maybe custom diffPrint methods for condition lists that default to "unified" diffs?

[brodieG]

Leaning towards just showing the diffs in full no matter what.