Continue after prepare failure in tmt try

Useful if machine needs tweaks before prepare can work, e.g. enable
repos.

Fix: #2685

tmt/trying.py

@@ -338,7 +338,11 @@ def action_start_test(self, plan: Plan) -> None:
 
         plan.discover.go()
         plan.provision.go()
-        plan.prepare.go()
+        try:
+            plan.prepare.go()
+        except GeneralError as error:
+            self.print(str(error))
+            return
         plan.execute.go()
 
     def action_start_login(self, plan: Plan) -> None:
@@ -349,7 +353,11 @@ def action_start_login(self, plan: Plan) -> None:
         self.action_start(plan)
 
         plan.provision.go()
-        plan.prepare.go()
+        try:
+            plan.prepare.go()
+        except GeneralError as error:
+            self.print(str(error))
+            return
         assert plan.login is not None  # Narrow type
         plan.login.go(force=True)
 
@@ -446,7 +454,10 @@ def action_prepare(self, plan: Plan) -> None:
         Prepare the guest
         """
 
-        plan.prepare.go(force=True)
+        try:
+            plan.prepare.go(force=True)
+        except GeneralError as error:
+            self.print(str(error))
 
     def action_execute(self, plan: Plan) -> None:
         """