[MP] Fix #369 - siege menu shaders not working

The problem was due to the new API not replicating exactly the functionality
given by trap_R_RegisterShaderNoMip in the UI module as there was some
extra code to load the shader name from a cvar if an asterisk was the
first character of the shader.

Thanks to redsaurus on the help with this.
[MP] Improved error reporting of DLL load failure

Added missing error code argument, added error reporting to client binary,
and display the additional information when developer is non-zero.
Merge branch 'master' of https://github.com/JACoders/OpenJK