Throw error if gridpack used and gridpack not present in directory

Changes introduced

• Throw error in logParser if gridpack is used in the generation and no gridpack file is present in the DSID directory

Tests

Madgraph

I used the already existing DSID: 500000/500371 and I renamed the gridpack file to mc_13TeV...tar.gz1 (to just make it fail). Here is what it prints:

Powheg

With the new implementation based on the channges by Timothee:

Issues resolved

Closes #95 (closed)

Review

The implementation is based on what @zmarshal suggested in #69 (closed) and on the discussion with @tpelzer in #95 (closed) so I assign the MR to them. Once you are happy feel free to approve (or modify the branch as you like). Thanks!