Handle nonexistent libraries better

[remexre]

Right now, if you try to call a function from a library that doesn't exist, you get an infinite loop somewhere. Instead, this should throw.

[skeeto]

This is certainly not intentional. What might be happening is the subprocess is crashing and Emacs is hung up waiting for a response. If it's the C++ program stuck in a loop, and you can't find the specific source of this problem, manually construct the "bytecode" that triggers this and examine it under GDB without Emacs present.