warning for %%timeit empty cell

Notifies user if they run %%timeit with an empty cell block - see issue #2862.
warnings for empty cells with %%sx, %%system, or %%!

Warns the user if they run a %%sx derivative with an empty cell - issue #2862. This is not perfect since, from within sx, there's no way to to tell which of sx/system/! were issued and we have to be generic.
remove magic-specific empty cell warnings
general empty cell warning
replace warning with UsageError
new style string formatting + explicit error message
new style formatting for magic-not-found warning
Merge pull request #2874 from rlmv/emptycell

Empty cell warnings