Improve postfit error message

Created by: J-M-Moore

Currently when trying to run postfit on new fits naively, the error message:

[WARNING]: Cannot find info file at /rds/user/jmm232/hpc-work/nnpdf_dev_install/top_fits/no_top_fit/nnfit/no_top_fit.info
[ERROR]: Error in postfit:
Postfit cannot find a valid LHAPDF info file

is given. It would be useful if this error message was more descriptive and pointed to the appropriate tutorial in the docs.