Recover from errors in individual rows -- serialize incomplete conversion

[albertmeronyo]

Sometimes COW will fail at converting a file (e.g. Unicode errors, out of memory, etc.). Currently serialization only happens at the end of conversion, so an exception will produce no output (even if most of the file was processed correctly).
A try/except mechanism on a per-row basis would help at reporting these errors and producing an output conversion, even if incomplete.