These errors did not stop the script
You might want to set $ErrorActionPreference in the script or use –ErrorAction on the commandlet
http://blogs.technet.com/b/heyscriptingguy/archive/2010/03/09/hey-scripting-guy-march-9-2010.aspx
Cannot find path 'filelocation' because it does not exist
In my environment, the SQL Server Agent runs as a domain login (example MyDomain\svc-sql), I have found that a powershell step in a SQL Server job can have permission problem with folders, including the default backup folder. I find that I need to explicitly set the folder permission to allow the service account read/write/modify as needed (right click folder, properties, security...)
It appears that when the Agent makes the backup file with a job step running T-SQL it runs as MSSQLSERVER but when it uses powershell step to access the file it only has the MyDomain\svc-sql permissions.
Cannot call method
I think this is result of previous error (failure of the 'Set-location')