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
```

Start by reproducing the encrypt call with the KMSMasterKeyProvider example and a disabled or deleted CMK. Trace how the DisabledException becomes EncryptKeyError, then add or update coverage so encryption reports the appropriate error for the affected CMK instead of the two errors shown.

Written by the indexing model from the issue text.