WATCH during MULTI shouldn't fail transaction #3009

[tishun]

Closes #3009

Problem statement
The submit attempts to solve two distinct issues existing in the driver currently:

1. A neseted WATCH (WATCH called after a MULTI and before an EXEC would result in some of the results in the TransactionResult to be missing, because the error result from the WATCH would push them out
2. A nested WATCH behaves differently than a nested MULTI, which instead throws an exception (but does not fail the transaction).

In a broader context the Redis server seems to handle errors inside transactions in three different ways:

• fails the transaction (if for ex. a malformed command is provided inside a transaction)
• does not fail the transaction, but includes errors in the output after EXEC (in case the command returns an error response, e.g. CLUSTER SLOTS called in a non-clustered environment)
• does not fail the transaction, but does not include the error in the output after EXEC (nested MULTI and WATCH)

Proposed solution

Make both WATCH and MULTI behave the same way when called inside a transaction:

• throw an exception to notify the application of an invalid state
• allow, if the exception is handled, to continue executing the transaction

Benefits

• this would constitute the minimal amount of deviation from the existing contract.
• smallest amount of change (not touching the code path of MULTI at all)

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• You submit test cases (unit or integration tests) that back your changes.