Check for available size before download/install

[ppadmavilasom]

@YustasSwamp reports this. Check for available size before download/install and throw appropriate errors if not enough space to perform action.

[oliverkurth]

There are two issues to address:

• space for downloading the package before doing the download. We can just check the space for the partition where we store the downloads (/var/tdnf/cache unless configured differently).
• space for the actual install. This can be tricky if there are multiple partitions involved.

[prestonsn]

PR #321 provides a cache directory space check. However, it does not address a potential out of disk space error during RPM installs.

[oliverkurth]

I think we can close this. The first item is addressed. The second is actually also addressed - it will be detected by the rpm backend, when the transaction is attempted.