API param validation: Add wrong value to error message on user params

It is always nice to get the wrong value back to know it.
Refactor a bit to avoid the array wrapping and unwrapping for non-multi
params.

Adjust another variable, add doc comments, removed a empty line and
reorder a condition for easy reading.

Change-Id: Ia91aa5908b82ad1209dc7da1139f91e1f2b45fac
Merge "API param validation: Add wrong value to error message on user params"