Check validity of arguments, better error messages.

[fredrikekre]

This segfaults Julia 1.0 so should not be merged until that is fixed, or until Julia 1.0 support is dropped.

[fredrikekre]

Hmm. Tests segfault on master with Julia 1.0.4 (https://github.com/fredrikekre/jlpkg/runs/196739829) even though there have been no changes to Julia code from https://github.com/fredrikekre/jlpkg/runs/148565014 so maybe this PR is not the problem.

[codecov-io]

Codecov Report

Merging #6 into master will decrease coverage by 4.47%.
The diff coverage is 0%.

@@           Coverage Diff           @@
##           master    #6      +/-   ##
=======================================
- Coverage   89.47%   85%   -4.48%     
=======================================
  Files           2     2              
  Lines          19    20       +1     
=======================================
  Hits           17    17              
- Misses          2     3       +1

Impacted Files	Coverage Δ
src/cli.jl	0% <0%> (ø)	⬆️
src/jlpkg.jl	100% <0%> (ø)	⬆️

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