Rename ErrorMessage and ErrorReason to FailureMessage and FailureReason

[noamran]

This issue is a follow up on- kubernetes-sigs/cluster-api#1670.

There is a lot of confusion when talking about these errors and why they're fatal errors. One possible solution is to improve the godoc around these fields and rename them to align them to the expected phase.

Once the rename is merged, a similar rename should be done in this provider.

[noamran]

/assign