Fix defining "10.2" as an explorer distribution alias

When defining "10.2" as a distribution alias, it returns a 404 while using "10.X" or such works alright.

This is linked to some kind of shortcut in the UI calls to be able to avoid using the distribution name, but it should not result in a 404 when none is found, and should continue "normal" resolution including aliases.

It also revealed, while debugging, that the Resource404 class handling redirections to the 404 page is actually throwing an error (luckily resulting in showing the 404 page...) introduced by commit https://github.com/nuxeo/nuxeo-explorer/commit/7826c9ccdc6849aba5a6ea0ff89d3e79af1dcd4e

Last but not least the distribution is retrieved several times in UI methods: this could be optimized.