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GH-36860: [C++] Report CMake error when system Protobuf exists but system gRPC doesn't exist (#36904)

### Rationale for this change

We require system gRPC when system Protobuf is found to avoid underlying library (Abseil) mismatch. We should report it as an error on `cmake...` time instead of build time.

### What changes are included in this PR?

Always call `find_package_handle_standard_args()` to report an error for the case. 

### Are these changes tested?

Yes.

### Are there any user-facing changes?

Yes.
* Closes: #36860

Authored-by: Sutou Kouhei <kou@clear-code.com>
Signed-off-by: David Li <li.davidm96@gmail.com>