Incorrect error message when DB is not writable

[amcinnes]

When the sqlite DB is not writable (e.g. read-only file permissions) it fails to create the since_table and logs "since tables already exists" (

logstash-input-sqlite/lib/logstash/inputs/sqlite.rb

Line 85 in cb332d2

@logger.debug("since tables already exists")

).

It would be easier to diagnose the problem if this situation is detected and logged as something informative like "Couldn't open database for writing".