common: fix the macros for malformed_input::what()

common: fix the macros for malformed_input::what()

the thrown exception of malformed_input should carry the function name in
which it was thrown. but what we have now is something like:
"buffer::malformed_input: __PRETTY_FUNCTION__ unknown encoding version >
100"
because __PRETTY_FUNCTION__ is not a macro any more. see
- https://gcc.gnu.org/onlinedocs/gcc-3.1/gcc/Function-Names.html
- https://gcc.gnu.org/onlinedocs/gcc/Function-Names.html
and it is not a string literal, so neither can we can concat it with
the literal err message.

Signed-off-by: Kefu Chai <kchai@redhat.com>