Improve retry loop(s) errors

[coryan]

@dbolduc noted:

observation: in the (rare) exhausted before start case, the error message is a bit redundant:

Retry policy exhausted in <location>:  Retry policy exhausted before first request attempt

I am fine with this.

Originally posted by @dbolduc in #12301 (comment)

and @devbww noted that we repeat "in" in the input to RetryLoopError().

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I think we need to create separate functions to return different errors, e.g.:

Status RetryLoopErrorNonIdempotent(Status s, char const* location);
Status RetryLoopErrorFinal(Status s, char const* location, bool exhausted);

In the body of these functions we can implement the formatting as needed.

We should also consider adding information in ErrorInfo() to indicate the type of failure in some kind of machine-consumable form.