Per SPEC-340, we can rely on libmongoc to merge multiple write and write concern error messages into the bson_error_t message, which is populated by a failed write execution.

While the bson_error_t message is statically allocated to ~500 bytes, that should be sufficient for a meaningful error message. Users can always inspect the attached WriteResult for full error information. This will allow us to delete code that concatenated error messages (PHPC-506).