Log NoLedgerException on debug level (#3117)

Log NoLedgerException on debug level (#3117)

NoLedgerException does not signify an error in the Bookie that needs
to be fixed. Instead it is - at most - a user error that the user is
notified about via the status code ENOLEDGER.
Logging this problem at error level introduces an odd difference
between the behavior of readLac using v2 versus v3 protocol version.
In the former case ReadEntryProcessor logs the same problem at debug
level. As a result changing protocol version appers to be introducing
an error.