[CLI] "libman init" command doesn't validate default provider or default path parameters

[alexgav]

Functional impact

It's possible to accidentally create a libman.json with an invalid default provider and destination

Minimal repro steps

1. Execute

libman init -p foo -d \toplevel\blah

Expected result

Error message stating invalid provider and/or destination

Actual result

File is created with invalid provider id and destination

Further technical details

libman install validates provider and destination, so I would think libman init would as well.

[jodavis]

\toplevel\blah is okay, but we should give an error for an invalid path, e.g. one that includes illegal characters