Fix typo in docs
Add `run` variant with stdout and stderr separated

The code for storing stdout and stderr separately is taken
from answer [1] by TheConstructor [3] to question [2].

[1] http://stackoverflow.com/a/18086548
[2] http://stackoverflow.com/questions/11027679/bash-store-stdout-and-stderr-in-different-variables
[3] http://stackoverflow.com/users/1266906/theconstructor
Make `load` handle absolute paths too

Add 2 simple tests on this.
Print failed statement in backtrace

Update tests accordingly.