Detect when a dependency was accidentally already invoked

mg.Deps(MyDep())

Normally this causes a nil pointer panic when we invoke the function

Error: runtime error: invalid memory address or nil pointer dereference

Now it will print the following as soon as its detected

Error: A dependency of the current target was defined improperly, with parenthesis. Dependencies should be defined as mg.Deps(MyDep), not mg.Deps(MyDep())

I have applied this change to checkfn so that it applies to serial and
parallel deps.

Signed-off-by: Carolyn Van Slyck <me@carolynvanslyck.com>

The new mg.F function is the root of the nil-pointer error that can happen
when someone accidentally calls mg.Deps(TargetA()). I have fixed the check
for whether a function was passed so that it won't panic and included
the usage for mg.F in the error message.

I then added a similar check, is the target a function, to
Deps/SerialDeps/CxtSerialDeps functions so that we can give a better
error message when it's passed a non-function. The error message
otherwise will reference mg.F which I don't think will help point people
to the real problem, which is how they called mg.Deps/SerialDeps.

Signed-off-by: Carolyn Van Slyck <me@carolynvanslyck.com>