Bug 23075: Better error handling in Mana config

Bug 23075: Better error handling in Mana config

Currently, if you input an incorrect mana service URL in your config
(http rather than https for example) the error message you are given is
the direct output of a failed json parse.  We should be able to catch
such failures and display a more meaningful error to the end user.

This patch makes it display a more friendly message.

Signed-off-by: David Nind <david@davidnind.com>
Signed-off-by: Nick Clemens <nick@bywatersolutions.com>
Signed-off-by: Martin Renvoize <martin.renvoize@ptfs-europe.com>