Simplify RateLimitExceeded with no retry-duration

- Avoid passing negative durations in error cases
- Drop unused message
- Return a duration for a bad forwarded-for

service/src/main/java/org/whispersystems/textsecuregcm/controllers/AccountController.java

@@ -635,9 +635,13 @@ public class AccountController {
     }
 
     final String mostRecentProxy = ForwardedIpUtil.getMostRecentProxy(forwardedFor)
-        // Missing/malformed Forwarded-For, cannot calculate a reasonable backoff
-        // duration
-        .orElseThrow(() -> new RateLimitExceededException(Duration.ofHours(-1)));
+        .orElseThrow(() -> {
+          // Missing/malformed Forwarded-For, so we cannot check for a rate-limit.
+          // This shouldn't happen, so conservatively assume we're over the rate-limit
+          // and indicate that the client should retry
+          logger.error("Missing/bad Forwarded-For, cannot check account {}", uuid.toString());
+          return new RateLimitExceededException(Duration.ofHours(1));
+        });
 
     rateLimiters.getCheckAccountExistenceLimiter().validate(mostRecentProxy);
 

service/src/main/java/org/whispersystems/textsecuregcm/controllers/RateLimitExceededException.java

@@ -4,22 +4,26 @@
  */
 package org.whispersystems.textsecuregcm.controllers;
 
+import javax.annotation.Nullable;
 import java.time.Duration;
 import java.util.Optional;
 
 public class RateLimitExceededException extends Exception {
 
-  private final Optional<Duration> retryDuration;
+  private final @Nullable
+  Duration retryDuration;
 
-  public RateLimitExceededException(final Duration retryDuration) {
-    this(null, retryDuration);
+  /**
+   * Constructs a new exception indicating when it may become safe to retry
+   *
+   * @param retryDuration A duration to wait before retrying, null if no duration can be indicated
+   */
+  public RateLimitExceededException(final @Nullable Duration retryDuration) {
+    super(null, null, true, false);
+    this.retryDuration = retryDuration;
   }
 
-  public RateLimitExceededException(final String message, final Duration retryDuration) {
-    super(message, null, true, false);
-    // we won't provide a backoff in the case the duration is negative
-    this.retryDuration = retryDuration.isNegative() ? Optional.empty() : Optional.of(retryDuration);
+  public Optional<Duration> getRetryDuration() {
+    return Optional.ofNullable(retryDuration);
   }
-
-  public Optional<Duration> getRetryDuration() { return retryDuration; }
 }