Provide meaningful error messages on authn JWT failures

[matthiasjung]

Desired Outcome

Replace current error message:
Unable to unmarshal token : unexpected end of JSON input
with:
./summon-conjur abc/def
ERROR Failed creating a Conjur client: 401 Unauthorized.

Implemented Changes

Minor update to file client.go checking HTTP Error code.

Connected Issue/Story

No issue reported so far.

Definition of Done

At least 1 todo must be completed in the sections below for the PR to be
merged.

Changelog

• The CHANGELOG has been updated, or
• This PR does not include user-facing changes and doesn't require a
  CHANGELOG update

Test coverage

• This PR includes new unit and integration tests to go with the code
  changes, or
• The changes in this PR do not require tests

Documentation

• Docs (e.g. READMEs) were updated in this PR
• A follow-up issue to update official docs has been filed here: [insert issue ID]
• This PR does not require updating any documentation

Behavior

• This PR changes product behavior and has been reviewed by a PO, or
• These changes are part of a larger initiative that will be reviewed later, or
• No behavior was changed with this PR

Security

• Security architect has reviewed the changes in this PR,
• These changes are part of a larger initiative with a separate security review, or
• There are no security aspects to these changes

[szh]

Thank you for the contribution @matthiasjung! I've gone ahead and added some tests and a changelog entry. Please let me know if it looks good to you and we'll get this merged.

[matthiasjung]

Hi @szh, thanks for enhancing changelog and adding tests cases!
Looks good now. Please go ahead.

[szh]

LGTM, thanks so much for the PR!