Add reassuring error message if call to get status bitstring fails.

[jchartrand]

If the http call to get the bitstring for the credential status fails (for whatever reason - timeout, internet connection, server error), show some kind of reassuring message that makes clear that this doesn't necessarily mean that the credential has been revoked or is invalid, just that something technical is preventing a conclusive check.

So maybe a message something like:

"The verification system is experiencing technical difficulties - please try again later. This does not mean your credential isn't valid, only that the system can't confirm just now."

Related to: openwallet-foundation-labs/learner-credential-wallet#551