Provide a better error message for non-existing gcpTempLocation

gcpTempLocation will default to using the value for tmpLocation, as long
as the value is a valid GCP path. Non-valid GCP paths are silently
discarded.

This change removes existence validation from the default value logic
such that downstream validation can provide a better error message.

runners/google-cloud-dataflow-java/src/test/java/org/apache/beam/runners/dataflow/DataflowRunnerTest.java

@@ -18,6 +18,7 @@
 package org.apache.beam.runners.dataflow;
 
 import static org.apache.beam.sdk.util.WindowedValue.valueInGlobalWindow;
+import static org.hamcrest.Matchers.both;
 import static org.hamcrest.Matchers.containsInAnyOrder;
 import static org.hamcrest.Matchers.containsString;
 import static org.hamcrest.Matchers.equalTo;
@@ -255,6 +256,26 @@ public void testPathValidation() {
     }
   }
 
+  @Test
+  public void testPathExistsValidation() {
+    String[] args = new String[] {
+        "--runner=DataflowRunner",
+        "--tempLocation=gs://does/not/exist",
+        "--project=test-project",
+        "--credentialFactoryClass=" + NoopCredentialFactory.class.getCanonicalName(),
+    };
+
+    try {
+      TestPipeline.fromOptions(PipelineOptionsFactory.fromArgs(args).create());
+      fail();
+    } catch (RuntimeException e) {
+      assertThat(
+          Throwables.getStackTraceAsString(e),
+          both(containsString("gs://does/not/exist"))
+              .and(containsString("does not exist or is not writeable")));
+    }
+  }
+
   @Test
   public void testPathValidatorOverride() {
     String[] args = new String[] {

sdks/java/core/src/main/java/org/apache/beam/sdk/options/GcpOptions.java

@@ -198,7 +198,7 @@ public String create(PipelineOptions options) {
       if (!Strings.isNullOrEmpty(tempLocation)) {
         try {
           PathValidator validator = options.as(GcsOptions.class).getPathValidator();
-          validator.validateOutputFilePrefixSupported(tempLocation);
+          validator.verifyPath(tempLocation);
         } catch (Exception e) {
           // Ignore the temp location because it is not a valid 'gs://' path.
           return null;

sdks/java/core/src/test/java/org/apache/beam/sdk/options/GcpOptionsTest.java

@@ -126,6 +126,16 @@ public void testDefaultGcpTempLocationInvalid() throws Exception {
     assertTrue(isNullOrEmpty(options.getGcpTempLocation()));
   }
 
+  @Test
+  public void testDefaultGcpTempLocationDoesNotExist() throws Exception {
+    GcpOptions options = PipelineOptionsFactory.as(GcpOptions.class);
+    String tempLocation = "gs://does/not/exist";
+    options.setTempLocation(tempLocation);
+    String msg = "gcpTempLocation should accept non-existing default tempLocation so downstream "
+        + "validation can provide a better error message";
+    assertEquals(msg, tempLocation, options.getGcpTempLocation());
+  }
+
   private static void makePropertiesFileWithProject(File path, String projectId)
       throws IOException {
     String properties = String.format("[core]%n"