Prevent two podman machines running on darwin

As issue #25112 points out, it was possible to start a machine on one of the darwin providers and then switch providers and start another one with a different name.  This PR firstly prevents that use which is a forbidden use case.

Secondarily, performed some minor cleanup on the error messages being used so that the error would be specific to this condition.

This bug fix is for darwin only.  In the case of Windows, we probably need to answer the question I raised in #24067 first, which is whether we want to stop allowing WSL to run multiple machines.

Fixes #25112

Signed-off-by: Brent Baude <bbaude@redhat.com>

pkg/machine/define/errors.go

@@ -8,12 +8,9 @@ import (
 )
 
 var (
-	ErrNoSuchVM         = errors.New("VM does not exist")
-	ErrWrongState       = errors.New("VM in wrong state to perform action")
-	ErrVMAlreadyExists  = errors.New("VM already exists")
-	ErrVMAlreadyRunning = errors.New("VM already running or starting")
-	ErrMultipleActiveVM = errors.New("only one VM can be active at a time")
-	ErrNotImplemented   = errors.New("functionality not implemented")
+	ErrWrongState      = errors.New("VM in wrong state to perform action")
+	ErrVMAlreadyExists = errors.New("VM already exists")
+	ErrNotImplemented  = errors.New("functionality not implemented")
 )
 
 type ErrVMRunningCannotDestroyed struct {
@@ -49,3 +46,16 @@ type ErrIncompatibleMachineConfig struct {
 func (err *ErrIncompatibleMachineConfig) Error() string {
 	return fmt.Sprintf("incompatible machine config %q (%s) for this version of Podman", err.Path, err.Name)
 }
+
+type ErrMultipleActiveVM struct {
+	Name     string
+	Provider string
+}
+
+func (err *ErrMultipleActiveVM) Error() string {
+	msg := ""
+	if err.Provider != "" {
+		msg = " on the " + err.Provider + " provider"
+	}
+	return fmt.Sprintf("%s already starting or running%s: only one VM can be active at a time", err.Name, msg)
+}