Explain shouldn't panic on invalid specs (#960)

* Explain shouldn't panic on invalid specs

Author: ptnapoleon

api/v1beta1/disruption_types.go

@@ -885,6 +885,18 @@ func (s DisruptionSpec) DisruptionCount() int {
 
 // Explain returns a string explanation of this disruption spec
 func (s DisruptionSpec) Explain() []string {
+	if err := s.Validate(); err != nil {
+		retErr := []string{"We were not able to explain this spec as it is not valid."}
+
+		if merr, ok := err.(*multierror.Error); ok {
+			for _, e := range merr.Errors {
+				retErr = append(retErr, e.Error())
+			}
+		}
+
+		return retErr
+	}
+
 	var explanation []string
 	explanation = append(explanation, "Here's our best explanation of what this spec will do when run:")