Makes the error messages report on RHSs and LHSs that do not match the
expected values when the relations are defeq. If the relations are not
defeq, the error message now no longer mentions the value of the whole
`calc` expression.
Adds a field to `mkCoe` with an optional callback to use to generate
error messages.
Note: it is tempting to try to make use of expected types when
elaborating the `calc` expression, but this runs into issue #2073.
Closes#4318
