Suggestion for warning if invalid annotation directory is passed

[zlskidmore]

If an incorrect annotation dir is given the getAnnoDir() function will first attempt to see if an ideal directory exist, if it doesn't it will look for additional directories annodir/readlength.* however if no readlength.* is present the code will fail with an uninformative error message along the lines of:

Error in dirs - readlength : non-numeric argument to binary operator

The offending line of code is: https://github.com/chrisamiller/copyCat/blob/1bbbf5b0bdf9d7e5f2977687607b22143e79f01e/R/functions.R#L451

and occurs because dirs is an empty list. It might be helpfull to add a line of code before this checking that annodir/readlength exists

[chrisamiller]

I'll add it to my list - PRs welcome! ;)

[zlskidmore]

sounds good! more of a note more than anything but happy to make a PR