Fix Blaze crash with bad Starlark flags and --keep_going.

https://github.com/bazelbuild/bazel/blob/28442ae70099812f57bd4db052b5cc63ecb31845/src/main/java/com/google/devtools/build/lib/runtime/StarlarkOptionsParser.java#L196
crashes with a NoSuchElementException on

    $ blaze build //foo --//flags:flag_target_that_doesnt_exist --keep_going

This is because `--keep_going` stops
skyframeExecutor.loadTargetPatternsWithoutFilters from throwing an exception
even though it produces no results.

PiperOrigin-RevId: 346131655
diff --git a/src/test/java/com/google/devtools/build/lib/starlark/StarlarkOptionsParsingTest.java b/src/test/java/com/google/devtools/build/lib/starlark/StarlarkOptionsParsingTest.java
index 994004a..42ef9a8 100644
--- a/src/test/java/com/google/devtools/build/lib/starlark/StarlarkOptionsParsingTest.java
+++ b/src/test/java/com/google/devtools/build/lib/starlark/StarlarkOptionsParsingTest.java
@@ -89,6 +89,19 @@
   }
 
   @Test
+  public void testBadFlag_keepGoing() throws Exception {
+    optionsParser.parse("--keep_going");
+    scratch.file("test/BUILD");
+    reporter.removeHandler(failFastHandler);
+
+    OptionsParsingException e =
+        assertThrows(OptionsParsingException.class, () -> parseStarlarkOptions("--//fake_flag"));
+
+    assertThat(e).hasMessageThat().contains("Error loading option //fake_flag");
+    assertThat(e.getInvalidArgument()).isEqualTo("//fake_flag");
+  }
+
+  @Test
   public void testSingleDash_notAllowed() throws Exception {
     writeBasicIntFlag();