[FIXED JENKINS-24676]: Give the user a readable error message when testing the connection instead of stack trace if their credentials are incorrect.
Various fields, especially lastFetchInstance, should be transient as they are fetched from EC2 anyway.
Fixed bug that caused infinite loop of NullPointerExceptions during provisioning of windows instances.

Signed-off-by: Stefan C. Mueller <scm@smurn.org>
Merge pull request #113 from smurn/master

Fixed bug that caused infinite loop of NullPointerExceptions
Merge pull request #112 from jglick/transient-lastFetchInstance

[JENKINS-24938] Various fields should be transient
Merge pull request #108 from recampbell/JENKINS-24676

[FIXED JENKINS-24676]: Give the user a readable error message
Merge branch 'master' into JENKINS-25209