v1::exception and v1::server_error (CXX-3237, CXX-3238)

[eramongodb]

Resolves CXX-3237 and CXX-3238 for the v1::exception and v1::server_error components.

As described in #1482 and prior PRs, rather than exposing error categories in the v1 API, the mongocxx::v1::source_errc error condition is used to describe the origin of error codes. See the "throw_exception" test case in mongocxx/test/v1/exception.cpp for reference. On the off-chance that a bson library error code is returned by the mongoc API, its code is stored in the error message ("bson error code <code>: <message>") and converted into a MONGOC_ERROR_BSON_INVALID error code instead. (Note: bsoncxx::v1 exceptions are not converted.)

There is the possibility that the mongoc API returns a SASL library error code. This is probably both unlikely and not intentional given how the _mongoc_cyrus_is_failure function is implemented (mongoc should do its best to convert the SASL error code into a mongoc error code). Nevertheless, if it does manage to be returned by the mongoc API, it is converted into a (undocumented) sasl_error_category error code for diagnosis. When an error category has a dedicated error code type, std::error_code::message(v) formats the error code message using that type, e.g. "mongoc_error_code_t:<code>". Otherwise, the message is formatted as "<source> error code <code>", e.g. "server error code 12345".

The v1::exception::internal::throw_exception() overloads are the primary method by which most errors returned by mongoc will be converted into mongocxx::v1 exceptions. These overloads are comparable to v_noabi::throw_exception<exception_type>() in mongocxx/mongoc_error.hh; unlike the v_noabi overloads, the v1 overloads determine the exception class to throw according to the provided arguments. This is the primary means by which CXX-834 will be addressed going forward, e.g. if we decide to implement the write-related error classes from the CRUD specification, the throw_exception() function may be extended to select the appropriate write error class to throw according to the fields present in the error document.

[kevinAlbs]

LGTM. I left minor test comments and a question.