Update duplicated 'when' clause warning message

[kddnewton]

Previously for duplicated when clauses like:

1: case foo
2: when :bar
3: when :baz
4: when :bar
5: end

You would get a warning like:

warning: duplicated 'when' clause with line 2 is ignored

which is supposed to read as in:

"The 'when' clause at line 4 duplicates (conflicts) with (a 'when' clause at) line 2. This is ignored."

The issue is that it can be read as the clause on line 2 is ignored. This commit updates the warning message to instead say:

warning: 'when' clause on line 4 duplicates 'when' clause on line 2 and is ignored

This makes it clear which clause is begin duplicated and which clause is being ignored.

This commit fixes both parsers.

[Edwing123]

At first I thought the one on line 4 was being ignored, I think something like the following is an option, too:

warning: 'when' clause on line 2 is ignored due to duplicated 'when' clause on line 4.

Or

warning: 'when' clause on line 2 is ignored because it's duplicated by 'when' clause on line 4.

[kddnewton]

The one on line 4 is ignored, the first one wins.

[Edwing123]

The one on line 4 is ignored, the first one wins.

I see.