MINOR: more details in error message descriptions (#3267)

Reviewers: Guozhang Wang <wangguoz@gmail.com>

clients/src/main/java/org/apache/kafka/common/protocol/Errors.java

@@ -378,14 +378,14 @@ public enum Errors {
                 return new TopicExistsException(message);
             }
         }),
-    INVALID_PARTITIONS(37, "Number of partitions is invalid.",
+    INVALID_PARTITIONS(37, "Number of partitions is below 1.",
         new ApiExceptionBuilder() {
             @Override
             public ApiException build(String message) {
                 return new InvalidPartitionsException(message);
             }
         }),
-    INVALID_REPLICATION_FACTOR(38, "Replication-factor is invalid.",
+    INVALID_REPLICATION_FACTOR(38, "Replication factor is below 1 or larger than the number of available brokers.",
         new ApiExceptionBuilder() {
             @Override
             public ApiException build(String message) {