return the error when refresh fails, not the token

[serbrech]

Fix

This fixes the error message returned when refreshing the token fails. it currently returns the token :

failed to refersh token: &{{{ 0 0 0 } 0xcb74e8 {{ <nil> false } { <nil> false } {http <nil> 169.254.169.254 /metadata/identity
/oauth2/token false api-version=2018-02-01&client_id=XXXXX&resource=https%3A%2F%2Fservicebus.azure.net%2F } { 
<nil> false }} XXXXXXXXX https://servicebus.azure.net/ true 300000000000} 0xc0000cf800 0xc00017f740 <nil> [] 5}

changing it to return the actual error provides the right information:

 Error: Received unexpected error:
failed to refresh token: the MSI endpoint is not available. Failed HTTP request to MSI endpoint:
Get "http://169.254.169.254/metadata/identity/oauth2/token?api-version=2018-02-01": context deadline exceeded

Which in my case was a the IMDS endpoint timing out. This happens very easily when running a set of integration tests concurrently (just a dozen...), each authenticating to the IMDS endpoint. It seems that IMDS handles them very badly.

• [x] All tests passed
• [x] Add changes to changelog.md

Environment

• OS: Darwin Kernel Version 19.6.0
• Go version: 1.14

[jhendrixMSFT]

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