[Proposal] Improve reporting of open file limit issues

[vicsn]

💥 Proposal

The past month 3 node operators experienced opaque halts/sync issues reporting as previous state root mismatch, whereas the real issue was a lack of open file descriptors.

Currently we only issue a warning at startup about a draft limit. We should:

• turn the startup check into a panic
• periodically see if we can open a new file and report an ERROR if it fails.