improvements for error pages

[kpsherva]

Here are my latest thoughts:

• When a 500 error occurs on any endpoint, we would intercept it on base.js:23, and trigger a (new) action
• A component placed at the level of BackOffice.js:13 would have a conditional display of the main content depending on the value of this action. If everything goes well, the children are displayed - nothing changes. In case of an error it would replace the pane content by the 500 error page. Similar pattern on the frontsite
• That way we ensure that if something is wrong the interactive part of the interface disappears. But most importantly we don't need to implement fine grained logic for each page as it would be handled globally
• This could be extended to HTTP errors other than 500; I can think of 403, 404 and 429

It would essentially look like the following with the corresponding error page displayed (again, same for the frontsite):

Originally posted by @FlorianCassayre in #176 (comment)

[FlorianCassayre]

Also check that inveniosoftware/invenio-app-ils#213 is implemented (backend).