Fix the calls of webob exception classes

The webob exception classes has "explanation" arguments but some
methods calls the classes with invalid arguments. This patch fixes
them.
In addition, this patch changes the error handling of agent_build
_create() because the method raises AgentBuildExists exception only.
The API of v3 has already done it.

Change-Id: I2af5aae297c09ed34fadf6520f1a06228624a5a9
Merge "Fix the calls of webob exception classes"