Error on `@throws` to non-methods (align with Scala 2)

[tanishiking]

Related to #26802 (pointed out by @SolalPirelli )

Currently, Scala 3 accepts @throws annotations on any places. However @throws is effective only on methods/constructors, and otherwise it's ignored.

Scala 2 already rejects @throws on non-methods/constructors:

import java.io.IOException
@throws(classOf[IOException])
class C
// error: @throws only allowed for methods and constructors

Proposal

Reject @throws earlier aligned with Scala 2.

Scala 2: scala/scala#9465

[SolalPirelli]

This seems like a broader issue with our annotations, e.g., we don't check @switch either. :/