[mlir] reword error message on unloaded dialect (#67980)

The previous message was confusing as it mentioned "registration" but
isn't in fact related to dialect registration. Use other words instead.

Author: ftynse

mlir/include/mlir/IR/Builders.h

@@ -477,8 +477,8 @@ class OpBuilder : public Builder {
     if (LLVM_UNLIKELY(!opName)) {
       llvm::report_fatal_error(
           "Building op `" + OpT::getOperationName() +
-          "` but it isn't registered in this MLIRContext: the dialect may not "
-          "be loaded or this operation isn't registered by the dialect. See "
+          "` but it isn't known in this MLIRContext: the dialect may not "
+          "be loaded or this operation hasn't been added by the dialect. See "
           "also https://mlir.llvm.org/getting_started/Faq/"
           "#registered-loaded-dependent-whats-up-with-dialects-management");
     }