this is in preparation for #4542, and extracts from `findRecArg` the
functionality for trying one particular argument.
It also refactors the code a bit. In particular
* It reports errors in the order of the parameters, not the order of
in which they are tried (it tries non-indices first).
* For every argument it will say why it wasn't tried, even if the
reason is quite obviously (fixed prefix, or `Prop`-typed etc.)
Therefore there is some error message churn.
18c97926a1