API change: error -> status for exception handling

Instead of the trivial structure SH_Error, SH_Status is used. The name
was chosen, because error/status is set independently whether an error
has occurred. Beside the error type, it also contains the associated
errno and an error message. The error message is also printed, when it
is set. Generating error messages with variadic arguments is now also
supported.
There are also macros to check for a set status.

The exception handling was removed for the *_free methods, because they
can't fail predictably during runtime.

Unfortunately the compiler reports, that inside the macro set_status
printf may be called with NULL [printf (NULL)], although, this is
explicitly debarred.