Change the disk assembly to raise exceptions

This big patch converts the bdev Assemble() methods and the supporting
functions to raise exceptions instead of returning False. This is a big
patch, since the assembly functions touch other functions: add children,
creation, etc. However, the patch does not add much new code, rather it
reworks existing code.

One of the biggest changes is in the rework of the DRBD8._SlowAssemble()
method (one of the most complicated/ugly ones). Hopefully the new
version is a little bit more readable.

Reviewed-by: ultrotter