Fixed run_migrations.sh as part of work to fixup the onramp process. 

The soda2.conf file moved from onramp/ into docs/onramp/ when the onramp repo was merged into the docs repo. I did the work to update the path that run_migrations.sh looks to find this file. I also made the script fail with a helpful error message if it doesn't find it.
If you have SODA_CONFIG set to override the location of the conf file, then this will fail unnecessarily if you don't checkout docs.