Catch exceptions when finding commands on PATH in Prompt.Shell
DynamicBars-use-ExtensibleState

Hooks.DynamicBars was previously using an MVar and the unsafePerformIO hack (
http://www.haskell.org/haskellwiki/Top_level_mutable_state ) to store bar
state. Since ExtensibleState exists to solve these sorts of problems, I've
switched the file over to use unsafePerformIO instead.

Some functions' types had to be changed to allow access to XState, but the
public API is unchanged.