Add linters from https://github.com/gostaticanalysis

[ldez]

I propose to introduce the following linters:

• https://github.com/gostaticanalysis/nilerr: finds code that returns nil even though it checks that error is not nil.
• https://github.com/gostaticanalysis/forcetypeassert: finds forced type assertions.