Requirement conflicts could be better communicated

[bovlb]

allwpilib/wpilibNewCommands/src/main/java/edu/wpi/first/wpilibj2/command/ParallelCommandGroup.java

Line 53 in 1db3936

"Multiple commands in a parallel composition cannot require the same subsystems");

Traditionally, it was fairly easy to track down problems with conflicting requirements just by looking at the back trace. With modern tools (e.g. PathPlanner) that create commands for you, it can be much harder. (See related issue mjansen4857/pathplanner#601)

I suggest that this exception message be augmented to include the name of the command, the name of the command being added, and the names of the subsystems in conflict. Perhaps something like:

throw new IllegalArgumentException(
  String.format("Problem adding command %s to %s because subsystems %s are already required.  " 
    + "Multiple commands in a parallel composition cannot require the same subsystems", 
    command.getName(), this.getName(), requirementsIntersectionString(command.getRequirements()));