Quickfix download and cran checks

[MartinHinz]

We were asked to make the quicksetup function fail graceful if oxcal is not downloadable. I implemented the required features, as described in #43, in this branch.

[MartinHinz]

Since you, @nevrome, originally have written this part, could you please be so kind as to check my alterations?

[codecov-commenter]

Codecov Report

Merging #44 (c335c5e) into master (2578090) will decrease coverage by 0.36%. The diff coverage is 56.25%.

@@            Coverage Diff             @@
##           master      #44      +/-   ##
==========================================
- Coverage   66.59%   66.22%   -0.37%     
==========================================
  Files          13       13              
  Lines         898      909      +11     
==========================================
+ Hits          598      602       +4     
- Misses        300      307       +7     

Impacted Files	Coverage Δ
R/utility_functions.R	80.00% <56.25%> (-7.50%)	:arrow_down:

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[nevrome]

Hm - shouldn't this error then also be handled in quickSetupOxcal? Without an internet connection this user facing function still fails rather unpleasantly:

Oxcal doesn't seem to be installed. Downloading it now:
trying URL 'https://c14.arch.ox.ac.uk/OxCalDistribution.zip'
Error Downloading OxCalDistribution.zip:
URL 'https://c14.arch.ox.ac.uk/OxCalDistribution.zip': status was 'Couldn't resolve host name'
No internet connection or data source broken?

Error in setOxcalExecutablePath(exe) : No file at given location 

If downloadOxcal fails, then the function should stop there without running setOxcalExecutablePath.

[MartinHinz]

Thank you for pointing this out! Actually, I philosophically would expect an error here, and not a rather silent warning thingy. I can not understand the CRAN Policy at this point. Never the less, f7be88e introduces additional catch for this situation? Is that ok for you?