Do not accept invalid uncertaintiy combinations

[JeroenDeDauw]

From @mzeinstra:

EDTF humanises some string that cannot exist within EDTF. only ~, ? OR % allowed/ no combinations allowed examples of not allowed/no EDTF 1985-04-12% and 1985-04-12?~ and 1985-04%~

[mzeinstra]

To be complete the reference says:

the uncertain and approximate qualifiers, '?' and '~', when applied together, are combined into a single qualifier character '%';

So we can have the two qualifiers, they should be presented by %