Skip pretty formatting if the first line isn't a TAP plan

Closes #21

test/bats.bats

@@ -150,3 +150,10 @@ fixtures bats
 
   [ "$tap_output" != "$pretty_output" ]
 }
+
+@test "pretty formatter bails on invalid tap" {
+  run bats --tap "$FIXTURE_ROOT/invalid_tap.bats"
+  [ $status -eq 1 ]
+  [ "${lines[0]}" = "This isn't TAP!" ]
+  [ "${lines[1]}" = "Good day to you" ]
+}

test/fixtures/bats/invalid_tap.bats

@@ -0,0 +1,7 @@
+echo "This isn't TAP!"
+echo "Good day to you"
+exit 1
+
+@test "truth" {
+  true
+}