fix: report the actual function name in the non-angle Unit error of trig functions

[greymoth-jp]

The shared trigUnit helper hard-codes cot in the "is no angle" error, so every function that uses it (sin, cos, tan, sec, csc, and cot) reports cot when it is given a unit that is not an angle. Five of the six name the wrong function:

math.sin(math.unit('5 celsius'))
// TypeError: Unit in function cot is no angle

That should read "function sin". The message already follows the pattern "Unit in function is no angle", so the name just needs to come from the function that was actually called instead of being hard-coded to cot. The fix reads it from self.name, which resolves to sin, cos, tan, sec, csc, or cot depending on which function the signature ends up in.

All six trig functions had an "invalid unit" test that only checked that an error was thrown, never the message, so the wrong name went unnoticed. I tightened those assertions to also check the message. The five non-cot tests fail on the current code and pass with this change; cot was already correct.