Badly formed exercises that throw docutils.utils.SystemMessage will cause crash

[David Coles]

Badly formed exercises such as ch5_ambiguous.xml should not crash IVLE. The SystemMessage exception of docutils needs to be caught and inform the user of the error in the exercise. Otherwise any use of this exercise will crash the worksheet or exercise viewer it is called from.

[David Coles (dcoles)]

Something a simple as ' a) Part A' (note the leading space) in the reStructuredText is enough to trigger this error.

Ideally we should print the exercise as having an error and continue on with the rest of the worksheet.

[David Coles (dcoles)]

Fixed in r1676. Show the error if we can't pass the reStructuredText - likely just a small syntax error.