[MERGE] Warn when setting push location is masked by locations.conf (#122286)

[Ian Clatworthy]

Ian Clatworthy has voted +1.
Status is now: Approved
Comment:
Minor gripe emailed to the list but it doesn't need to be changed to 
merge.

For details, see: 
http://bundlebuggy.aaronbentley.com/request/%3C468BB614.4080108%40utoronto.ca%3E