[dotest.py] Present a better error when lldb module can't be found.

We try to run lldb -P and jump through a bunch of other hoops
to figure out where the Python module is so that we can add its
location to sys.path.  The error message printed when this
doesn't work was left over from the time when Mac was the only
platform, so it presented a Mac-specific diagnostic on all
platforms related to the LLDB Framework.  This patch prints
a more useful diagnostic when not on Mac that gives a number
of other diagnostic messages to check.

llvm-svn: 234049