Build Log: Error message corrected. Launch Step: fixed ability to open step with incorrect plugin settings

Fixes https://github.com/qubell/contrib-jenkins-qubell-plugin/issues/32 and https://github.com/qubell/contrib-jenkins-qubell-plugin/issues/20

![image](https://cloud.githubusercontent.com/assets/3889581/3431246/de8d63f0-0068-11e4-9756-d1327beb5441.png)
Credentials error on edit build step.

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![image](https://cloud.githubusercontent.com/assets/3889581/3431285/2827c8c0-0069-11e4-9f87-0959584fb578.png)
Invalid credentials Build Log error

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![image](https://cloud.githubusercontent.com/assets/3889581/3431326/9034bd56-0069-11e4-8b43-57862181e09e.png)

Missing parameters Build Log Error (Previously "Requested manifest version does not exist")