[runners/converter] Reject conversion queries without source unit
Commit 6e6c5707 made it so that "1/2" gets correctly rejected (used to match in certain locales such as German), but broke unit tests by matching invalid fraction queries like "4/4>mm". With this commit, both of these invalid queries get rejected. Also adding more fraction tests with units left out.
Showing with 33 additions and 14 deletions