ENH dwifslpreproc: Write eddy_quad error to output directory

[Lestropie]

If eddy_quad fails, then a text file is written in the scratch directory containing the stdout / stderr contents. However if -nocleanup is not specified, this file is lost, as the script will nevertheless execute to completion. Given that the output directory for the eddyqc_* options is always created (it just won't be complete if eddy_quad is unsuccessful, it will contain only information from eddy), should be easy to write that text file into that directory on eddy_quad failure, so that its failure can potentially be diagnosed.