Improve pruneMode.String()

[AskAlexSharov]

Current version is from E2. Need change it to --prune.mode=archive/full/minimal

Now i see:

EROR[02-28|14:44:15.467] Erigon startup                           err="not allowed change of --prune flag, last time you used: --prune.h.older=100000"
not allowed change of --prune flag, last time you used: --prune.h.older=100000

[VBulikov]

VBulikov added Imp2 as Importance

[awskii]

also when starting and erigon notices that flag has changed, message printed incorrectly with empty "previous" flag