Encrypt throws incorrect error when a CMK within KMSMasterKeyProvider is disabled

If a CMK is disabled/deleted within KMSMasterKeyProvider CMKs, the encrypt call throws a couple of errors (below).

> botocore.errorfactory.DisabledException: An error occurred (DisabledException) when calling the Encrypt operation: arn:aws:kms:eu-west-2:xxxxxxx

> aws_encryption_sdk.exceptions.EncryptKeyError: Master Key arn:aws:kms:eu-west-2

```
import aws_encryption_sdk

kms_key_provider = aws_encryption_sdk.KMSMasterKeyProvider(key_ids=[
'arn:aws:kms:us-east-1:2222222222222:key/22222222-2222-2222-2222-222222222222',
'arn:aws:kms:us-east-1:3333333333333:key/33333333-3333-3333-3333-333333333333'
])
my_plaintext = b'This is some super secret data! Yup, sure is!'

my_ciphertext, encryptor_header = aws_encryption_sdk.encrypt(
source=my_plaintext,
key_provider=kms_key_provider,
encryption_context={
'not really': 'a secret',
'but adds': 'some authentication'
}
)

decrypted_plaintext, decryptor_header = aws_encryption_sdk.decrypt(
source=my_ciphertext,
key_provider=kms_key_provider
)

assert my_plaintext == decrypted_plaintext
assert encryptor_header.encryption_context == decryptor_header.encryption_context
```