fix: Improve error handling of non-existent/non-accessible filepaths

[ahl27]

Found a small bug, figured it would be faster to include a fix for it with the report.

Running rcmdcheck on a filepath that doesn't exist produces an uninformative error:

> rcmdcheck::rcmdcheck("this path is not a real path!")
## Error in if (file.info(path)$isdir) { : 
##  missing value where TRUE/FALSE needed

This is because file.info(path) returns all NA values if path is either non-existent or not readable. I encountered this when I accidentally had a small typo in the path to the package I wanted to test.

This fix wraps the file.info(path)$isdir call with some other error handling to produce more informative errors (and potentially allow for other handling / custom error messages later). It also handles the case where the file/directory exists, but isn't readable. Also included a unit test.

New version:

> rcmdcheck::rcmdcheck("this path is not a real path!")
## Error in safecheck_isdir(path) : path 'this is a test' does not exist!

[ahl27]

tests seem to be failing due to a CRAN error, not because of changes in this PR.

[ahl27]

added extra_cols = FALSE argument to file.info, since we only need res$isdir and this can slightly improve performance.