Correctly returning data from tx execution through module

[remedcu]

The execTransactionFromModuleReturnData(...) returned data from guard execution, if a guard was enabled. It was rectified by assigning the returnData to the return value from the execute(...) and then running the post-tx checks in the guard. As these pre and post-checks through guards were shared between all execTransactionFromModule*(...), internal functions were written for the same.

Closes #708

[coveralls]

Pull Request Test Coverage Report for Build 7503212266

• 0 of 0 changed or added relevant lines in 0 files are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage increased (+0.04%) to 94.591%

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Totals
Change from base Build 7476408467:	0.04%
Covered Lines:	400
Relevant Lines:	408

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💛 - Coveralls

[mmv08]

Looks good to me. Let's also wait for @nlordell and @akshay-ap reviews, as I'd deem this a security-critical change and the more eyes, the better

[nlordell]

LGTM, just left one OCD nit 😛.