Added NotFound exception to improve error messages in cases when entity is not found.
Merge branch 'cb-78' into errors
Updated messages.pot
Improved styling of error pages.
Moved navbar template into a separate file.
Updated messages.pot
Showing basic error info on 500.html
Fixed link to "About" page in navbar.
Revert "Showing basic error info on 500.html"

This reverts commit 62c6410a466a6a4d78ed40febff1ef642a03a785.
Moved line break in 500.html