tree-wide: panic when json marshalling fails

According to the new guidelines in docs/errors.md.

Note that this does not include code that marshals to a writer that
might fail (when a connection drops, for example).

internal/blueprint/blueprint.go

@@ -43,13 +43,13 @@ type Group struct {
 func (b *Blueprint) DeepCopy() (Blueprint, error) {
 	bpJSON, err := json.Marshal(b)
 	if err != nil {
-		return Blueprint{}, err
+		panic(err)
 	}
 
 	var bp Blueprint
 	err = json.Unmarshal(bpJSON, &bp)
 	if err != nil {
-		return Blueprint{}, err
+		panic(err)
 	}
 	return bp, nil
 }

internal/store/store.go

@@ -287,7 +287,6 @@ func (s *Store) change(f func() error) error {
 	if s.stateChannel != nil {
 		serialized, err := json.Marshal(s)
 		if err != nil {
-			// we ought to know all types that go into the store
 			panic(err)
 		}