Improve error handling of invalid values in import plugins

[Nicklas Nordborg]

The following was sent to the BASE mailing list (GPR import plugin error) by david vilanova:

In some cases the "log ratio (...)" field has some values that are named "error". This come from the fact that when the background noise is higher than the intensity of the expressed oligo it is not possible to compute the log ration so genepix will return "error" as value for this specific field. Well, base is returning an error when trying to import such values. Could that be fixed ?? I want to keep this files so the option of ignoring this lines is not a solution since i want to keep all the lines of the GPR file.

We need an option of inserting null values for values that fails and not just skipping the whole line.