Reporting correct error message on HTTP errors for Emscripten target

[nhachicha]

Currently, Http errors report as

[json.exception.parse_error.101] parse error at line 1, column 1: syntax error while parsing value - unexpected end of input; expected '[', '{', or a literal

This fixes the network transport for Emscripten to report errors properly

{"error":"password must be between 6 and 128 characters","error_code":"BadRequest","link":"https://realm.mongodb.com/groups/5d84ec2dcf09a2dcb343aa7a/apps/63d93da2dad7c0ff7cabdd5f/logs?co_id=6480656e2dac9d38f50821a5"}. Client Error: 400

[elle-j]

Thanks, this is helpful!