[MERGE] Give nicer errors for some write operations on read-only transports.

[Robert Collins]

Robert Collins has voted reject.
Status is now: Vetoed
Comment:
This is the wrong approach to the problem. On local disk we will 
encounter read only problems from time to time. LBYL should be avoided, 
instead handle the problem when you fail to write more accurately.

For details, see: 
http://bundlebuggy.aaronbentley.com/request/%3C20070902160411.242522900DD%40twitch%3E