Improve error reporting on mg.Deps() usage errors

[flowchartsman]

Given the following magefile:

// +build mage

package main

import (
        "github.com/magefile/mage/mg"
        "github.com/magefile/mage/sh"
)

func Echo() error {
        return sh.RunV("echo", "hello")
}

func EchoDep() error {
        mg.Deps(mg.F(Echo()))
        return nil
}

$ mage echo
hello

$ mage echoDep
hello
Error: runtime error: invalid memory address or nil pointer dereference

[flowchartsman]

I'm a moron. That should be

mg.Deps(mg.F(Echo))

[natefinch]

Hmmm, yes, but we should have a better error message for that, since this is likely to be a common mistake. Panicking in that case is bad UX.

[flowchartsman]

Agreed, I'll change the issue to something more descriptive.

…

On Mon, Feb 1, 2021 at 10:05 AM Nate Finch ***@***.***> wrote: Reopened #332 <#332>. — You are receiving this because you modified the open/close state. Reply to this email directly, view it on GitHub <#332 (comment)>, or unsubscribe <https://github.com/notifications/unsubscribe-auth/AAHCJJCRLFH4CREUZ6LOAVDS427M5ANCNFSM4W4AY6YQ> .

[flowchartsman]

To clarify for anyone who finds this issue: the error was the result was making the call to deps against a function result, not a function reference. The apparent success of the dependee function (Echo()) was simply that it was run for evaluation at its call site, rather than mage calling it as part of the normal dependency resolution process. Then mg.Deps() attempted reflection on the return value (or lack thereof) as opposed to the function reference itself.