Describe the problem

As per: examples we should be able to catch NetworkException when getting JWK using cached JwkProviderBuilder. But in here that error is caught and replaced with SigningKeyNotFoundException (as ExecutionException includes NetworkException):

@Override
public Jwk get(final String keyId) throws JwkException {
    try {
        String cacheKey = keyId == null ? NULL_KID_KEY : keyId;
        return cache.get(cacheKey, new Callable<Jwk>() {
            @Override
            public Jwk call() throws Exception {
                return provider.get(keyId);
            }
        });
    } catch (ExecutionException e) {
        throw new SigningKeyNotFoundException("Failed to get key with kid " + keyId, e);
    }
}

This basically recreates the issue: #79.

What was the expected behavior?

NetworkException being thrown directly from: JwkProviderBuilder(URL(...)).cached(...).build().get(...) method when unable to connect to the url.

Reproduction

• Create cached provider for some urlPath of an offline server: val provider = JwkProviderBuilder(URL(urlPath)).cached(10, 1, TimeUnit.MINUTES).build()
• Try to access the JWK for some keyId: provider.get(keyId)
• Get the SigningKeyNotFoundException: Failed to get key with kid ****
• Only able to trace exception.cause: java.util.concurrent.ExecutionException: com.auth0.jwk.NetworkException: Cannot obtain jwks from url ****

Environment

• Version of this library used: 0.21.2
• Version of Java used: 11 (Kotlin 1.3.72)
• Other modules/plugins/libraries that might be involved:
• Any other relevant information you think would be useful: