Better error message of method_from_call() on S3 methods

[DanChaltiel]

Hi,

method_from_call() works only on S4 methods and not on S3 methods.

Indeed, the doc says "find corresponding S4 method", but it never explicitly tells us it cannot find S3 methods.

If this is a technical limitation, I think it would be worth mentioning it in the doc, maybe in a @details section (and likewise if it is a WIP).

Moreover, it is not evident for a naive user (like me) whether a method is S3 or S4, and the error message is not very helpful:

pryr::method_from_call(mean(1:10))
#> Warning in formals(fun): argument is not a function
#> Error in getGeneric(f, !optional): no generic function found for 'mean'

^{Created on 2023-02-04 with reprex v2.0.2}

Note that this reprex is oddly only really reproducible inside reprex(): in a fresh R session, a mean() call will work, but an anova() call will not.