Determine how we want to handle "undefined behaviour"

[hugovdm]

Various "meaningless" corner cases are possible with unit conversions. Some examples:

• Use of temperature in mixed units
• Use of prefixes with temperatures (units with offsets)

We don't currently fail for such cases. To be determined:

• Do we want to check for these cases and return errors?
• Do we want to leave this behaviour undefined?
  • If undefined, do we want to add unit tests that demonstrate the current behaviour? (And thereby arguably "locks in" the undefined behaviour? Or leave in comment-out or otherwise-disabled unit tests that show the current behaviour?)

[sffc]

Discussion:

• @younies suggests treating square Celsius or Fahrenheit as a relative quantity, in which case we can safely drop the offset. Or we could add separate units or another API parameter. @hugovdm compares this to date vs. duration, where we should have separate types.
• @sffc thinks we should use an error code here since it isn't clear what the correct behavior should be. @hugovdm suggests setting the error code while constructing the UnitConverter.