fix: returning comparable error on contex's cancelled errorcase

[ermyar]

Returing wrapped context.Cause(ctx) error in <-ctx.Done() case. Allows compare errors using errors.Is/As.

Fixes #457

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

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

[oleg-jukovec]

Please, validate and update other places of a context usage:

$ grep "ctx.Done" -r connection.go
		case <-ctx.Done():
		case <-ctx.Done():
	case <-ctx.Done():

[bigbes]

Single comment, otherwise LGTM.

[oleg-jukovec]

Please, squash commits into a single one according to the rules:

https://www.tarantool.io/en/doc/latest/contributing/developer_guidelines/#how-to-write-a-commit-message

[oleg-jukovec]

Please, rebase the branch on the master branch and resolve conflicts.

[oleg-jukovec]

Please, update a link to an issue in the commit message and in the PR message:

Fixes (457)

->

Fixes #457