feat: handle port forward subdomain conflict

internal/workspace/errors.go

@@ -1,9 +1,19 @@
 package workspace
 
+import "strings"
+
 type errWorkspaceExists struct {
 	message string
 }
 
+type errPortMappingConflicts struct {
+	conflicts []string
+}
+
 func (err *errWorkspaceExists) Error() string {
 	return err.message
 }
+
+func (err *errPortMappingConflicts) Error() string {
+	return "Subdomain(s) already in use: " + strings.Join(err.conflicts, ", ")
+}