[HOTFIX] When loading template, ignore the element that is not found.

Regarding this issue (https://github.com/AIFDR/inasafe/issues/865), currently if we choose template that does not have some elements, it will throw an error if we open it on composer or will not generate the map report if choose to open on pdf.

@timlinux proposed that we should just ignore the missing element on the template (or the custom template). This is the implementation of that idea.

However, if that case happens on this implementation, it will open (composer) / generate (print to pdf) a not making sense map. User does not get the idea of what should be the correct element id that present on the template; They need to look the documentation. Even though I implemented this PR, I am still not all in with this idea. Please @timlinux test and review this PR.