handle unreachable urls in download.file.wcheck

* identical is all very well, but http_header is a char vector not a list
Ignore download test when flybrain unreachable
Better error handling in download.file.wcheck

* specifically for case when no internet available
Improve docs for download.file.wcheck
Fix new bugs in download.file.wcheck when url accessible
improve test messages for downloads
always return path when file available, fix update logic

* was always downloading ...
Test for use of cached files as appropriate
depend on nat>=1.5.14 for nlscan
Bump version to 0.6