Modernize error handling

[tzdybal]

• more idiomatic approach to errors (var instead of struct)
• use errors.Is, as it's preferred way to check errors in go 1.13+

Reference: https://blog.golang.org/go1.13-errors

[tzdybal]

@musalbas please make a final decision (see: https://github.com/lazyledger/smt/pull/21#pullrequestreview-600201662)

[musalbas]

Isn't this the wrong naming convention for sentinel value based errors? From the docs you linked, sentinel values start with Err.