feat(useOptions.validate): add Validate function to use command

Did a bit of refactoring here.
Previously, if the `qri use` command is run without any arguments, the cmd Run function (not the useOptions.Run function), would print out the cmd.Help() text.
However, this breaks a few of our patterns.
First, on no other command do we print the help text if no arguments are given
Second, the Validate function would need to take a `*cobra.Command`. Since we are trying to modularize these functions (for clarity and for testing purposes), it seems like passing the Validate function the entire command violates the spirit of what we were doing in the last refactor.

cmd/use.go

@@ -28,10 +28,7 @@ func NewUseCommand(f Factory, ioStreams IOStreams) *cobra.Command {
 		},
 		Run: func(cmd *cobra.Command, args []string) {
 			ExitIfErr(o.ErrOut, o.Complete(f, args))
-			if o.Clear == false && o.List == false && len(args) == 0 {
-				err := cmd.Help()
-				ExitIfErr(o.ErrOut, err)
-			}
+			ExitIfErr(o.ErrOut, o.Validate())
 			ExitIfErr(o.ErrOut, o.Run())
 		},
 	}
@@ -60,6 +57,17 @@ func (o *UseOptions) Complete(f Factory, args []string) (err error) {
 	return
 }
 
+// Validate checks that any user input is valide
+func (o *UseOptions) Validate() error {
+	if o.Clear == false && o.List == false && len(o.Refs) == 0 {
+		return lib.NewError(ErrBadArgs, "please provide dataset name, or --clear flag, or --list flag\nsee `qri use --help` for more info")
+	}
+	if o.Clear == true && o.List == true || o.Clear == true && len(o.Refs) != 0 || o.List == true && len(o.Refs) != 0 {
+		return lib.NewError(ErrBadArgs, "please only give a dataset name, or a --clear flag, or  a --list flag")
+	}
+	return nil
+}
+
 // Run executes the search command
 func (o *UseOptions) Run() (err error) {
 	var (