[SPARK-17623][CORE] Clarify type of TaskEndReason with a failed task.

## What changes were proposed in this pull request?

In TaskResultGetter, enqueueFailedTask currently deserializes the result
as a TaskEndReason. But the type is actually more specific, its a
TaskFailedReason. This just leads to more blind casting later on – it
would be more clear if the msg was cast to the right type immediately,
so method parameter types could be tightened.

## How was this patch tested?

Existing unit tests via jenkins.  Note that the code was already performing a blind-cast to a TaskFailedReason before in any case, just in a different spot, so there shouldn't be any behavior change.

Author: Imran Rashid <irashid@cloudera.com>

Closes #15181 from squito/SPARK-17623.