Add `static final String err(String msg)` to Command abstract class.

[cbloodsworth]

This would have an implementation something like:

static final String err(String msg) {
   return this.name + ": " + msg;
}

This would allow us for commands to return standardized error messages based on their own name.

[JacobNickerson]

im on it gang

[cbloodsworth]

This issue has been resolved for a while -- I'm going to close the issue.