Activate error handling for graph build step

[leonardehrenfried]

The script actually has error handling but it's skipped when it's run with bash build-graph.sh.

This makes the build fail when the NeTex file cannot be downloaded, which is what we want (rather than silently continuing).

@dulvui

[leonardehrenfried]

As expected the graph build fails because the NeTEx file cannot be downloaded:

Downloading NeTEx transit data from ftp://ftp01.sta.bz.it/netex/2024/plan/EU_Profil/NX-PI_01_it_apb_LINE_apb__20240820.xml.zip
#=#=#                                                                         
##O#-#                                                                        
##O=#  #                                                                      
#=#=-#  #                                                                     
-#O#- #   #                                                                   
-=#=#   #   #                                                                 
curl: (9) Server denied you to change to the given directory

I think we should merge this anyway so we know when there is a problem downloading the NeTEx file.