Handle R CMD check crashes better

[gaborcsardi]

Now we throw an error if R CMD check exits with an
exit status greater than 1. (It exists with 1 if the
package fails to install, which we don't want to
error on.)

Closes #110.

[codecov-commenter]

Codecov Report

Merging #163 (4a340a6) into master (279fa1b) will decrease coverage by 0.47%.
The diff coverage is 58.82%.

❗ Current head 4a340a6 differs from pull request most recent head c42f338. Consider uploading reports for the commit c42f338 to get more accurate results

@@            Coverage Diff             @@
##           master     #163      +/-   ##
==========================================
- Coverage   81.57%   81.09%   -0.48%     
==========================================
  Files          19       19              
  Lines         993     1005      +12     
==========================================
+ Hits          810      815       +5     
- Misses        183      190       +7     

Impacted Files	Coverage Δ
R/package.R	88.29% <56.25%> (-6.83%)	⬇️
R/background.R	95.00% <100.00%> (ø)

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