Minor improvement in error messages

[etiennebacher]

Fix spacing when several strings in Err_plain(). Also, I don't understand what the "Some names seen" message means and I don't think it makes the error message better so I suggest we remove it.

Old:

> pl$when(a = 1)
Error: Execution halted with the following contexts
   0: In R: in pl$when():
   0: During function call [pl$when(a = 1)]
   1: Detected a named input.This usually means that you've used `=` instead of `==`.Some names seen:a

New:

> pl$when(a = 1)
Error: Execution halted with the following contexts
   0: In R: in pl$when():
   0: During function call [pl$when(a = 1)]
   1: Detected a named input. This usually means that you've used `=` instead of `==`.