Improvements to tool test error handling.

[jmchilton]

• Add integration test for invalid admin key.
• Add tool test to ensure error message is valid JSON.
• Swap order of require_admin and expose_api - expose_api sets environment so require_admin knows it should return JSON and not HTML. This is the bug fix.

How to test the changes?

(Select all options that apply)

• [x] I've included appropriate automated tests.

License

• [x] I agree to license these and all my past contributions to the core galaxy codebase under the MIT license.