Wrap last connection error

[avalchev94]

Hi all! First, I want to thank you for listening to the community and adding WithReturnConnectionError, it's really useful!
I have an idea how we can make it even better. Currently, the connection error's type is lost and only the error's text is used:

	conn, err = nil, fmt.Errorf("%v: %v", ctx.Err(), err)

At the end we have an error from type errors.errorString and the only way to check the underlying error is string comparison, which is unreliable.

Use case(s) - what problem will this feature solve?

• checking error type
• getting additional details from the custom error type

Proposed Solution

1. Easiest solution is using the %w modifier, which wraps the error. Later, that error can be unwrapped with errors.Unwrap:

	conn, err = nil, fmt.Errorf("%v: %w", ctx.Err(), err)

However, from what I see in go.mod, the minimum supported go version is 1.11 and that's a problem, because above features were added in 1.13

2. Creating custom error type which implements Unwrap interface from errors package. In that way:

• if user is using version >=1.13, errors.Unwrap can be used directly;
• if user is using version < 1.13, anonymous Unwrap interface can be defined and used for obtaining the underlying error;

 type wrapError struct {
     msg string
     err error
}

func (e *wrapError) Unwrap() error {
   return e.msg
}

// func (e *wrapError) Error() string

Sorry for the pseudo code, if that idea is approved, I can do a better version.

[dfawley]

This is fixed.