Improve ErrorClassification deserialization

Closes gh-1115

Author: rstoyanchev

spring-graphql/src/main/java/org/springframework/graphql/client/ResponseMapGraphQlResponse.java

@@ -177,7 +177,12 @@ public ErrorClassification getErrorType() {
 				return graphql.ErrorType.valueOf(classification);
 			}
 			catch (IllegalArgumentException ex) {
-				return org.springframework.graphql.execution.ErrorType.valueOf(classification);
+				try {
+					return org.springframework.graphql.execution.ErrorType.valueOf(classification);
+				}
+				catch (IllegalArgumentException ex2) {
+					return ErrorClassification.errorClassification(classification);
+				}
 			}
 		}