sample applications should not set exit_code as return statement of functions

As commented in https://github.com/ARMmbed/mbed-os-example-tls/pull/199#discussion_r224024552
internal functions return code should be set to a variable different than `exit_code`.
It's not a functionality bug, but it shows bad practice

The issue does not name files or tests. Start by reviewing the sample applications and the linked pull-request discussion, then identify internal functions that return through `exit_code`; done means those functions use a separate return-code variable while preserving application exit behavior.

Written by the indexing model from the issue text.