improve message when lumimask download from URL fails

[belforte]

current output is like

crab submit crab_data_djets_2017B.py
Will use CRAB configuration file crab_data_djets_2017B.py
Importing CMSSW configuration JetShapeJPTAnalysis_data_s_sql_dy_newcor_2017n.py
Finished importing CMSSW configuration JetShapeJPTAnalysis_data_s_sql_dy_newcor_2017n.py
The following user output files (not listed as PoolOuputModule or TFileService in the CMSSW PSet) will be collected: '13_TeV_doublemuons.root'
ERROR: URLError: <urlopen error [Errno -2] Name or service not known>

    Please email [email protected] for support with the crab.log file or crab.log URL.
    Client Version: v3.210825
    Please use 'crab uploadlog' to upload the log file /afs/cern.ch/work/k/kodolova/public/L1L2L3/L2L3RESIDUALS/CMSSW_11_0_3/src/WORK/crab_20211021_102007/crab.log to the CRAB cache.

which makes it almost impossible for users to figure out w/o bugging support.
see e.g.
https://hypernews.cern.ch/HyperNews/CMS/get/computing-tools/6199/1/1/2/1.html