Running SSIS package from SQL Agent job owned by a non-sysadmin domain user
The problem looks more complex than it is. Since you are using SQL 2014 you are probably being bitten by the new security features introduced in 2012.

The only thing that actually matters is:

Server: {server name}, Package path: \SSISDB\Foo\Bar\foobar.dtsx, Environment reference Id: NULL.   
Description: Login failed for user '{domain user that runs SSIS ETL overnight}'.


Your Proxy user's login most likely doesn't have access to the SSISDB catalog (even though he might have access to SQL Server).
You need to map the login to an SSISDB user and configure access to the SSISDB folders/projects in Integration Services.

Please have a look at this MSDN blog post SSIS Catalog Access Control Tips and SQL 2012 SSIS Catalog Permissions

Once you have the package actually loading you might run into other security context issues but you should get better logging from integration services itself.