hpx::error_code is not usable for lightweight error handling

[hkaiser]

hpx::error_code is not usable for lightweight error handling as it constructs an exception representing all kind of information about the execution environment at the point of construction of the error code. This is desirable to make hpx::error_code and hpx::exception semantically equivalent. At the same time, it is desirable to be able to use the error_code as a lightweight means of reporting error conditions in cases where no extensive execution environment informartion is necessary. We need to create such a facility which blends well with existing code, but which allows to inhibit the creation of the full fletched exception information while constructing the error_code.