PanicOnWarnings is unrecoverable when using gh-ost as library

[ggilder]

Despite the name, gh-ost doesn't actually use panic() when PanicOnWarnings is enabled. When the PanicOnWarnings flag is enabled on the Migrator instance, and an SQL warning occurs, the error flows through these channels:

1. terminateRowIteration sends error to rowCopyComplete
2. consumeRowCopyComplete forwards error to PanicAbort
3. listenOnPanicAbort calls Log.Fatale(err)
4. Fatale calls os.Exit(1) which kills the entire process

When using gh-ost as a library, this prevents the calling application from recovering from panics since the entire process is terminated.

[ggilder]

Resolved by 8bc63f0