Change const char * to std::string in BaseException

[IceMage144]

Signed off by: @victordomiciano
Signed off by: @Kazuo256
Signed off by: @josealvim

[Kazuo256]

It seems va_start on Windows cannot receive reference-type arguments. We can separate the code of the constructor in a helper function on system/exceptions.cc that actually receives a const char*, and the constructor just delegates the string formatting to it.

[henriquegemignani]

I'd suggest keeping a version that receives a const char*: compile time strings such as "foo" are of the type const char[4], so converting to a std::string and convert back seems like a waste.

[Kazuo256]

I was hoping to change the formatting code to variadic templates eventually, meaning this change would only be temporary and we'd soon be rid of the forth-and-back conversion.