return an error indication for invalid /labelValues filter parameters

[willr3]

Right now an invalid filter to <test>/labelValues returns the full list of labelValues as though filter was not used. This can lead to users incorrectly believing the results reflect the desired filter.

I beleive we should instead return an error to indicate that the provided filter was invalid. This will lead to correct filter usage rather than silently accepting invalid filters.

[lampajr]

If I am not wrong, this change was incorporated into #1977, therefore I am closing this one as completed.

Feel free to re-open it if I misunerstood something.