[imp #168] Reload pdf on non critical errors

Sometimes, the compilation of a document with \LaTeX produces some
errors, but a pdf file is still generated.
In such cases, the web client now loads the new pdf file, while still
indicating errors.
If no pdf file was produced, nothing changes.
Merge pull request #177 from satabin/feature/update-pdf-on-error

[imp #168] Reload pdf on non critical errors