Right now, the io warnings both print to stderr and write to the
state. That can be easily modified.
We also add a getWarnings function which pulls warnings out of the state
for instances of PandocMonad.
Errors can be thrown purely with `throwError`. At the moment there are
only three kinds of errors:
1. PandocFileReadError FilePath (for problems reading a file from the
filesystem)
2. PandocShouldNeverHappenError String (for stuff that should never
happen but we need to pattern-match anyway)
3. PandocSomeError String (a grab bag of everything else)
Of course, we need to subdivide the third item in this list.
This requires a default environment. The state variables are pretty
straightforward. The env variables are a little trickier. I'm just
making most of them empty for now. Note that some of them (like
defaultReferenceDocx/ODT) will be coming out soon anyway.
Until we fix fetchItem and fetchItem' to make use of MonadError, we have
to thow an exception. We'll throw PandocFileReadError. Note that this is
temporary.
