/combo/ endpoint throws 500 when interactants cannot be found

[xyzt_]

Correct behavior:

• 404 if no interactant is provided (bare /combo/)

• 400 if a single interactant is provided (/combo/a/ whereas a exists or not)

• 404 if a single valid interactant is provided (/combo/a+mdma/ where a does no exist)
  returns an useless empty page for now

• 404 if any interactant is invalid (/combo/a+mdma+lsd where a does not exist)
  for now, invalid interactants are ignored, if 2 valid interactants cannot be found 500 is throwed

  Writing unit tests for these can be a good idea.