Add faketime format exception

Signed-off-by: Aaron Lew <[email protected]>

Author: aaronlew02

tuf-cli/src/main/java/dev/sigstore/tuf/cli/TufConformanceServer.java

@@ -95,8 +95,16 @@ private static void handleExecute(HttpServletRequest request, HttpServletRespons
         System.setErr(errPs);
       }
 
-      Instant fakeNow = Instant.ofEpochSecond(Long.parseLong(executeRequest.faketime));
-      TestClock.set(Clock.fixed(fakeNow, ZoneOffset.UTC));
+      try {
+        Instant fakeNow = Instant.ofEpochSecond(Long.parseLong(executeRequest.faketime));
+        TestClock.set(Clock.fixed(fakeNow, ZoneOffset.UTC));
+      } catch (NumberFormatException e) {
+        throw new IllegalArgumentException(
+            "Invalid 'faketime' format. Must be a long representing epoch seconds, but was: '"
+                + executeRequest.faketime
+                + "'",
+            e);
+      }
 
       List<String> resolvedArgs = new java.util.ArrayList<>();
       resolvedArgs.addAll(Arrays.asList(executeRequest.args));