Improve exceptions about mixed types in Expressions.

[Keryn Knight]

The ​source which raises the exception has enough information to say both what types were found, and which of those were unexpected, and probably have a useful repr()

In the test suite, the unexpected output types encountered seem to be DurationField and IntegerField, so a more thorough message might be something like:
Expression repr(self) contained mixed types: DateField, DurationField. DurationField was unexpected; you must set the output_field= for this Expression to either DurationField(), DateField() or ... (??? I dunno, some concrete explanation of what the output_field has to be/implement if you're not going to use any of the builtins)

The merit of including the repr is arguable, as the Expression may not what the user put in (eg: in the test suite it always seems to be a CombinedExpression(lhs, connector, rhs)) but it gives more of a hint in a query which contains multiple expressions (either nested or separate) as to which one is actually causing the problem vs just being told "something was wrong. Put an output_field= everywhere until it ceases, your guess is as good as mine"; at the very least the word Expression could be replaced with the class name which is actually raising it.

[Mariusz Felisiak]

Agreed, we can add types to this exception but I would like to keep it simple, e.g. "Expression contains mixed types: FloatField, IntegerField. You must set output_field to IntegerField."

[Mariusz Felisiak]

​PR

[Mariusz Felisiak <felisiak.mariusz@…>]

In 34f8eeea:

Fixed #30548 -- Improved exception when expression contains mixed types.