[BUG] main module evaluates to DeferredFunctionCall

[line-o]

Describe the bug

Consider a main module returning the value of a function call. The module is called from the controller.xq providing it an HTML payload (POST request).

If this main module is called first - returning an expected value - it will, when called with parameters that will raise an expected error, evaluate to a DeferredFunctionCall instead of catching the XPathException.

Adding a return type to the function declaration fixes the issue.

Expected behavior

Whenever an error is raised inside a function the expression will not evaluate to any value of any type but the error is raised.

To Reproduce

1. checkout Next major version templating#17
2. run npm install
3. run npm test
4. all tests should pass
5. remove the return type from templates:process (https://github.com/eXist-db/templating/blob/7cbb1d0d907608ccd77268baaf02db3a84f185fd/content/templates.xqm#L164)
6. run npm test again
7. one test-case testing for an error fails now because exist handles the requests differently: parameter start is out of bounds(!)
8. re-run the mocha testsuite without updating the library npm run test:mocha
9. three test-cases fail (all with the parameter start is out of bounds problem)

Running this in a debuggable exist instance reveals that the error is not thrown but a DeferredFunctionCall is returned instead.
When exist tries to handle the returned value it fails to do so.

Context (please always complete the following information):

• OS: macOS 12.6
• eXist-db version: 6.1.0-SNAPSHOT
• Java Version 1.8.0_352-b08

Additional context

• How is eXist-db installed? built from source 6723ed9
• Any custom changes in e.g. conf.xml? none