Make GM_setValue throw a descriptive error, when called with insufficient arguments

[arantius]

Originally: DevjaVu ticket 44

If GM_setValue receives a single argument in produces an error message because the second argument is undefined. This is confusing if the name argument has been omitted, e.g.:

GM_setValue(GM_getValue("count", 0) + num);

The thrown error sends the debugger in the wrong direction, searching for places that undefined could creep into num.

Proposal: GM_setValue should test that arguments.length === 2 and throw an error if the assertion fails.

[erikvold]

if (arguments.length !== 2) {
  throw new Error("GM_setValue requires 2 arguments");
}

Look alright?

[avindra]

I think this is unnecessary.

[arantius]

Patch available, no obvious backwards compatibility issues. Tagged for v.next

[Martii]

Integrated via this commit.

Anthony you may want to actually notate this and always ensure that a label is always present on each closed issue yourself when merging upstream.

[arantius]