Use QProcess rather than system().

This makes it easier to read stdout and stderr, which we may want to
display at some later point.

It appears to be less useful for asciidoc, but it'll certainly be useful
for other input formats.
Trivial coding style cleanup
Display compile errors.

Show the output of the compilation process (if any) in a text box below
the HTML output.
Color error output in red, for addition visibility.