Clearer warning message

[ZevEisenberg]

Old warning message:

Do not use 'unimplemented' with '@DependencyClient'; it is a replacement and implements the same runtime functionality as 'unimplemented' at compile time

This was confusing because it was not clear what “it” referred to, nor what was a “replacement” for what.

New warning message:

Do not use 'unimplemented' with '@DependencyEndpoint'; the '@DependencyEndpoint' macro provides an 'unimplemented' default value for you.

I also added a period at the end, but I can remove it if it’s not consistent with project style.

[ZevEisenberg]

Updated with workshopped message.

[stephencelis]

Thanks!