check for available disk space before fetching distfile(s)

[radhermit]

Currently if a distfile being fetched is larger than the available space where it will be stored, the package will begin to be fetched and then fail through all the additional fetch requests once the storage limit is hit.

To solve this, the amount of available space should be checked before fetching and/or the fetch/build process should error out once it hits the first storage limit failure.