add Error_display method to _metakernel.py

[jld23]

add Error_display method that will allow an error to be thrown and also display non-text objects

In the SAS Kernel, based on metakernel, there are two streams of information -- the log and the list. In parsing the log you can determine if there is an error and lag the cell as such but in a log program there could be valid output (in the form of HTML) that is helpful to the user.

This commit adds a new method to not change existing behavior.

[coveralls]

Coverage decreased (-0.4%) to 75.526% when pulling 1991ec4ac64ac0b7c1d67ef896222f486efe5a83 on jld23:master into c0379fa80497f79972f38c5eb0f03db6631fafb0 on Calysto:master.

[dsblank]

This looks reasonable. Could you add an example in the example directory in lieu of a test?

[dsblank]

Thanks!