Switch reindex with slices error to IAE

If you try to reindex with multiple slices against a node that
doesn't support it we throw an `IllegalArgumentException` so
`assertVersionSerializable` is ok with it and so if this happens
in REST it comes back as a 400 error.

modules/reindex/src/main/java/org/elasticsearch/index/reindex/AbstractBulkByScrollRequest.java

@@ -424,7 +424,7 @@ public abstract class AbstractBulkByScrollRequest<Self extends AbstractBulkByScr
             out.writeVInt(slices);
         } else {
             if (slices > 1) {
-                throw new UnsupportedOperationException("Attempting to send sliced reindex-style request to a node that doesn't support "
+                throw new IllegalArgumentException("Attempting to send sliced reindex-style request to a node that doesn't support "
                         + "it. Version is [" + out.getVersion() + "] but must be [" + BulkByScrollTask.V_5_1_0_UNRELEASED + "]");
             }
         }

modules/reindex/src/test/java/org/elasticsearch/index/reindex/RoundTripTests.java

@@ -81,7 +81,7 @@ public class RoundTripTests extends ESTestCase {
 
         // Try slices with a version that doesn't support slices. That should fail.
         reindex.setSlices(between(2, 1000));
-        Exception e = expectThrows(UnsupportedOperationException.class, () -> roundTrip(Version.V_5_0_0_rc1, reindex, null));
+        Exception e = expectThrows(IllegalArgumentException.class, () -> roundTrip(Version.V_5_0_0_rc1, reindex, null));
         assertEquals("Attempting to send sliced reindex-style request to a node that doesn't support it. "
                 + "Version is [5.0.0-rc1] but must be [5.1.0]", e.getMessage());
 
@@ -105,7 +105,7 @@ public class RoundTripTests extends ESTestCase {
 
         // Try slices with a version that doesn't support slices. That should fail.
         update.setSlices(between(2, 1000));
-        Exception e = expectThrows(UnsupportedOperationException.class, () -> roundTrip(Version.V_5_0_0_rc1, update, null));
+        Exception e = expectThrows(IllegalArgumentException.class, () -> roundTrip(Version.V_5_0_0_rc1, update, null));
         assertEquals("Attempting to send sliced reindex-style request to a node that doesn't support it. "
                 + "Version is [5.0.0-rc1] but must be [5.1.0]", e.getMessage());
 
@@ -126,7 +126,7 @@ public class RoundTripTests extends ESTestCase {
 
         // Try slices with a version that doesn't support slices. That should fail.
         delete.setSlices(between(2, 1000));
-        Exception e = expectThrows(UnsupportedOperationException.class, () -> roundTrip(Version.V_5_0_0_rc1, delete, null));
+        Exception e = expectThrows(IllegalArgumentException.class, () -> roundTrip(Version.V_5_0_0_rc1, delete, null));
         assertEquals("Attempting to send sliced reindex-style request to a node that doesn't support it. "
                 + "Version is [5.0.0-rc1] but must be [5.1.0]", e.getMessage());