Properly report errors when converting integers

[GiovanniBussi]

The code is not behaving consistently when users do mistakes in the input such as passing a real instead of an integer. Plumed should just give an error in these cases.
See https://groups.google.com/d/msg/plumed-users/JTbvhtXQRx0/i95P189HXIYJ

[GiovanniBussi]

I had a quick look it seems not trivial. The difficulty is that we are using the same return value ("false") to detect when a number is missing and when it is wrong. I remove any milestone (probably it is better to change this in the master branch later)