Fix TypeScript Error in isConstructor

[SanderMuller]

This PR resolves a TypeScript error in the isConstructor utility function and improves error handling for enhanced type safety.

Key Changes

1. TypeScript Error Fix:

   • Resolved the TS18046: 'err' is of type 'unknown' error by explicitly checking whether the caught err is an instance of Error before accessing its message.

2. Improved Error Handling:

   • Updated the catch block to specifically handle cases where obj is not a valid constructor by checking the error message for "is not a constructor."

3. unknown instead of any

   • Using unknown instead of any aligns with TypeScript best practices by forcing developers to explicitly assert or narrow types, reducing potential runtime errors.

TS error TS18046 only happens when running TS in strict, so in the Echo project itself you don't see it until enabling strict, but our TS config falls over this library implementation, and the change is a net benefit IMO.