Need a Better Error Message when Key Length Does Not Match Algorithm

If the DirectEncrypter is used with a keylength that does not match the algorithm given, the message is very confusing. For example, when a 32-byte key (256-bit) key is used with A128GCM, you get this exception:

com.nimbusds.jose.JOSEException: The "A128GCM" encryption method is not supported by the JWE encrypter: Supported methods: [A256GCM, A128CBC-HS256, A128CBC+HS256]

The error made me think I didn't have the correct crypto provider installed, when in fact it's just a mismatch between the provided key and the expected key.

Near minimal Scala example:

import com.nimbusds.jose._
import com.nimbusds.jose.crypto._

class Test {
  private val key = {
    val random = new java.security.SecureRandom()
    val sharedSecret = new Array[Byte](32)
    random.nextBytes(sharedSecret)
    sharedSecret
  }

    val jwe = new JWEObject(new JWEHeader(JWEAlgorithm.DIR, EncryptionMethod.A128GCM), new Payload("Hello"))
    jwe.encrypt(new DirectEncrypter(key))
    jwe.serialize()
}

The last line (jwe.serialize) will throw the error above.

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