Hi VM,
In order to handle this you will have to create a calculated member named [fail rate percentage].
Before doing that, from the above mentioned fact table, you will have to create 2 measures Sum(Failures) and Sum(Attempts), which I suppose you already have.
Using these measures create a calculated member [Fail Rate Percentage]
Expression : [Measures].[Failures]/[Measures].[Attempts]
Format String : "Percent"
Below link would give insights on how to create calculated member.
http://technet.microsoft.com/en-us/library/ms166568.aspx
Remember calculated members are not stored physical in the cubes, they are calculated on the fly while querying the cube. And that is suffice in your case.
Saurabh Kamath