improve error message when multiple bean candidates are present and all are Secondary

[edgao]

e.g. if you have this code: (kotlin syntax, hopefully the intent is clear)

@Singleton
class SomeOtherBean(val foo: Foo)

interface Foo

@Singleton
@Secondary
class Foo1 : Foo

@Singleton
@Secondary
class Foo2 : Foo

micronaut throws a cryptic java.util.NoSuchElementException, because it blindly calls iterator.next().

This PR just adds a check for empty list, and throws a more informative exception.

(wasn't sure how to update the unit tests to verify this - DefaultBeanContextSpec didn't seem like the right place? I'd appreciate a pointer to the relevant test suite)

[CLAassistant]

All committers have signed the CLA.

[graemerocher]

could you add a test please

[edgao]

wasn't sure where to put tests 😅 is there an existing test suite? (DefaultBeanContextSpec didn't look right, but maybe I'm looking in the wrong place)

[graemerocher]

probably can go in test-suite submodule

[edgao]

done - verified locally that without the runtime diff, the test fails.

I couldn't figure out how to use e.getPossibleCandidates() - AFAICT it returned an empty iterator? So I just asserted on the exception message. Would appreciate a tip if there's a better way to do this, or if I'm instantiating the exception incorrectly.

[edgao]

@graemerocher is there anything else I should do here?

[edgao]

looks like CI is green! is this PR ready to go?