Add Nulls.fail() iterator

[nalimilan]

Provided by DataArrays as each_nafail(). It simply iterates over the
input and throws an error if a null is found.

[codecov-io]

Codecov Report

Merging #32 into master will not change coverage.
The diff coverage is 100%.

@@          Coverage Diff          @@
##           master    #32   +/-   ##
=====================================
  Coverage     100%   100%           
=====================================
  Files           1      1           
  Lines          50     53    +3     
=====================================
+ Hits           50     53    +3

Impacted Files	Coverage Δ
src/Nulls.jl	100% <100%> (ø)	⬆️

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[ararslan]

I've actually never understood the purpose of that function in DataArrays. It seems equally easy (and much clearer) to do

any(isnull, x) && throw(NullException())

explicitly in user code as needed.

[nalimilan]

I wanted to use exactly that construct in @deprecate, but I realized it would be slow if you actually expect there are no nulls in the data: then any(isnull, x) requires iterating over the whole vector, before likely doing that once again to do the actual operation. Better do the check on the fly.