Defer text generation in n2cq to avoid crash on empty history.
Extend n2c and n2cq to not crash on empty history.
Merge pull request #4 from MarcSchmitzer/development

n2c/n2cq fixes
Merge branch 'master' into development
Handle calls to snippets/actions with empty history

In order to reduce error handling in snippets/action, we introduce
a HistoryEmpty excpetion that is raised whenevery history.top is
accessed with an empty history. The MultiPicker then catches the
exception and tries to keep going.

If your snippet has any sideeffects that do not get collected when
the stack unwinds, you should really handle them on your by catching
the exception and then reraising.
Use correct variable name
More docs for the alchemy snippet