throw exception for invalid uuid

[rmarsollier]

Throw exception instead if an invalid uuid is given to tag() and untag().

I hope i did it the right way.

[coveralls]

Coverage decreased (-0.1%) to 46.101% when pulling 9c36afb on rmarsollier:master into a2c05b9 on MISP:master.

[codecov-io]

Codecov Report

Merging #56 into master will decrease coverage by -0.12%.
The diff coverage is 0%.

@@            Coverage Diff            @@
##           master     #56      +/-   ##
=========================================
- Coverage   46.21%   46.1%   -0.12%     
=========================================
  Files           7       7              
  Lines        1612    1616       +4     
=========================================
  Hits          745     745              
- Misses        867     871       +4

Impacted Files	Coverage Δ
pymisp/api.py	41.48% <0%> (-0.18%)	❌

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[Rafiot]

Yep, that works!