various error messages including extreme inputs and others

[lorenzwalthert]

This pull request closes #3 since it provides input checks and corresponding error messages for general case specified in #3 as well as tests for the two cases.

In addition, it provides a (preliminary) improvement to the following issues (see commit messages), mainly error handling:

• the argument labels is added for consistency with cut.default. This is desirable since if one wants to use cut for integers without being interested in standard labels, but because of method dispatch, cut.integer is called.
• providing a non-specific warning message saying how many missing values were generated, if any. Could be extended in the way that it is tracked back why that happened (i.e. NAs in x, breaks disjunct with x etc).
• sorting breaks if they are not and output warning with the new order.
• removing NAs in breaks and issue a warning.

[codecov-io]

Current coverage is 95.37% (diff: 100%)

Merging #4 into devel will increase coverage by 1.69%

@@              devel         #4   diff @@
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  Files             3          3          
  Lines            79        108    +29   
  Methods           0          0          
  Messages          0          0          
  Branches          0          0          
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+ Hits             74        103    +29   
  Misses            5          5          
  Partials          0          0          

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