Support attaching to processes that can't import madbg

[kmaork]

When attaching to a process, madbg injects code that tries to import madbg. If the target process doesn't have madbg in its sys.path, attaching will fail.
The solution is not as simple as adding madbg to the sys.path, as it has dependencies, and some of them might collide with already-loaded versions in the target process.
A potential solution is to try to create a standalone version of madbg, potentially using pex or a similar tool.
Another direction is for madbg to detect the target interpreter, check if madbg is installed, and if not, offer to install it before attaching.
Until any solution is ready, we should make sure that the error is clear and does not affect the target process (see #49).