Avoid std::exception inheritance by forge::Error class

[9prady9]

[9prady9]

https://stackoverflow.com/a/30760610/1985476

https://docs.microsoft.com/en-us/cpp/error-messages/compiler-warnings/compiler-warning-level-2-c4275?view=vs-2019

[9prady9]

It seems like in-lining the fg::Error completely in the header should resolve the issue. Of course, that alone won't cut it, all cpp implementations should become header only, which shouldn't be hard given that they are merely redirecting to C-API.