Return proper error when no snapshot files exist

[rlaisqls]

Added len(intervals) == 0 check in doBlockSnapshotsRangeCheck() function.
Returns descriptive error instead of panicking when no snapshot files exist.

# Before
$ ./build/bin/erigon seg integrity --datadir ~/Library/Erigon --failFast
...
panic: runtime error: index out of range [0] with length 0

# After
$ ./build/bin/erigon seg integrity --datadir ~/Library/Erigon --failFast
...
Error: no snapshot files found in /Users/.../Erigon/snapshots for type: headers

resolve #17227

[sudeepdino008]

hi. Thanks.
btw similar check is needed in checkIfStateSnapshotsPublishable

[rlaisqls]

Thank you, I’ve updated the code to include a similar check in checkIfStateSnapshotsPublishable.

[sudeepdino008]

LGTM