Make Go error handling consistent

[r4victor]

The error handling in dstack Go codebase is messy. There are at least three different ways errors are created and propagated:

1. gerrors
2. tracerr
3. Standard library (fmt.Errorf(..., %w, ...))

We should standardize on one approach and make it consistent. Using plain errors.New / fmt.Errorf should be the Go way but we lose file/line number info that comes with gerrors by default. tracerr prints full stacktrace unlike gerrors but it's a third party dep.