clarify the result of Connection.Do().Get()

[oleg-jukovec]

We add meaningful error descriptions for a read/write socket errors for and examples:

• ExampleConnection_Do demonstrates how to process a result.
• ExampleConnection_Do_failure demonstrates how to process a request failure.

I didn't forget about (remove if it is not applicable):

• Tests (see documentation for a testing package)
• Changelog (see documentation for changelog format)
• Documentation (see documentation for documentation style guide)

Related issues:

Closes #129