Fix: Executor overwrites Earthmover errors when using fallback encoding

[johncmerfeld]

Sometimes this happens:

• Input file has a bonkers encoding and is the wrong file
• First earthmover run fails to decode
• Retry earthmover with latin1, which is pretty much guaranteed to decode
• Second earthmover run fails because of the other problem (extra header, missing column, etc)
• We wrongly log the error as the initial encoding error

Doesn't display much incorrect in the main UI but does result in an incorrect stacktrace sent to the app.