Rework how error inspection and logging is done

Previous error reporting code was inconsistent in terms of what is
logged, when and by whom. This caused several problems such as: logs
missing important error context, duplicated error messages, too verbose
messages, etc.

Instead of logging all generated errors, module should log
only errors it 'handles' somehow and does not simply pass it to the
caller. This removes duplication, however, also it removes context
information. To fix this, exterrors package was extended to provide
utilities for error wrapping. These utilities provide ability to
'attach' arbitrary key-value ('fields') pairs to any error object while
preserving the original value (using to Go 1.13 error handling
primitives).

In additional to solving problems described above this commit makes logs
machine-readable, creating the possibility for automated analysis.
Three new functions were added to the Logger object, providing
loosely-typed structured logging. However, they do not completely
replace plain logging and are used only where they are useful (to allow
automated analysis of message processing logs).

So, basically, instead of being logged god knows where and when,
all context information is attached to the error object and then it is
passed up until it is handled somewhere, at this point it is logged
together with all context information and then discarded.

exterrors/fields.go

@@ -0,0 +1,51 @@
+package exterrors
+
+type fieldsErr interface {
+	Fields() map[string]interface{}
+}
+
+type unwrapper interface {
+	Unwrap() error
+}
+
+type fieldsWrap struct {
+	err    error
+	fields map[string]interface{}
+}
+
+func (fw fieldsWrap) Error() string {
+	return fw.err.Error()
+}
+
+func (fw fieldsWrap) Unwrap() error {
+	return fw.err
+}
+
+func (fw fieldsWrap) Fields() map[string]interface{} {
+	return fw.fields
+}
+
+func Fields(err error) map[string]interface{} {
+	fields := make(map[string]interface{}, 5)
+
+	for err != nil {
+		errFields, ok := err.(fieldsErr)
+		if ok {
+			for k, v := range errFields.Fields() {
+				fields[k] = v
+			}
+		}
+
+		unwrap, ok := err.(unwrapper)
+		if !ok {
+			break
+		}
+		err = unwrap.Unwrap()
+	}
+
+	return fields
+}
+
+func WithFields(err error, fields map[string]interface{}) error {
+	return fieldsWrap{err: err, fields: fields}
+}