fix: return error on invalid ref config

This "fixes" a panic that happens if the cmd or ref isn't defined for
a serial or parallel exec

Author: jahvon

internal/runner/parallel/parallel.go

@@ -70,6 +70,8 @@ func handleExec(
 			}
 		case refConfig.Cmd != "":
 			exec = execUtils.ExecutableForCmd(parent, refConfig.Cmd, i)
+		default:
+			return errors.New("parallel executable must have a ref or cmd")
 		}
 
 		execPromptedEnv := make(map[string]string)

internal/runner/serial/serial.go

@@ -63,6 +63,8 @@ func handleExec(
 			}
 		case refConfig.Cmd != "":
 			exec = execUtils.ExecutableForCmd(parent, refConfig.Cmd, i)
+		default:
+			return errors.New("serial executable must have a ref or cmd")
 		}
 		ctx.Logger.Debugf("executing %s (%d/%d)", exec.Ref(), i+1, len(serialSpec.Execs))