Verify disk space before starting backups (SYN-4578)

[Cisphyx]

No description provided.

[codecov]

Codecov Report

Base: 97.21% // Head: 97.11% // Decreases project coverage by -0.11% ⚠️

Coverage data is based on head (2943e46) compared to base (3155d78).
Patch coverage: 100.00% of modified lines in pull request are covered.

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #2990      +/-   ##
==========================================
- Coverage   97.21%   97.11%   -0.11%     
==========================================
  Files         218      218              
  Lines       43198    43188      -10     
==========================================
- Hits        41996    41940      -56     
- Misses       1202     1248      +46     

Flag	Coverage Δ
linux	97.11% <100.00%> (-0.01%)	⬇️
linux_replay	?

Flags with carried forward coverage won't be shown. Click here to find out more.

Impacted Files	Coverage Δ
synapse/lib/cell.py	98.26% <100.00%> (+<0.01%)	⬆️
synapse/tests/utils.py	94.46% <0.00%> (-1.89%)	⬇️
synapse/cortex.py	96.68% <0.00%> (-0.55%)	⬇️
synapse/lib/hiveauth.py	96.01% <0.00%> (-0.48%)	⬇️
synapse/lib/oauth.py	98.68% <0.00%> (-0.44%)	⬇️
synapse/lib/trigger.py	95.01% <0.00%> (-0.39%)	⬇️
synapse/lib/view.py	97.01% <0.00%> (-0.34%)	⬇️
synapse/cmds/cron.py	97.56% <0.00%> (-0.25%)	⬇️
synapse/lib/jsonstor.py	98.32% <0.00%> (-0.24%)	⬇️
synapse/axon.py	98.73% <0.00%> (-0.15%)	⬇️
... and 11 more

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[vEpiphyte]

We should be able to tell them how much disk space we currently detect available; in use and what our backup requirements are. That could make this message actionable.

[Cisphyx]

updated message to include free and required space

[vEpiphyte]

actionable exception