Add Hooks documentation, remove unexpected logs

Documentation/GettingStarted.md

@@ -727,6 +727,29 @@ You can recover from failure of observable by using `catch` operator. There are
 
 There is also `retry` operator that enables retries in case of errored sequence.
 
+### Default error handling
+
+If you don't provide `onError` callback default handling will be performed. `Hooks` enum is responsible for it. 
+
+It just prints received error in `DEBUG` mode and does nothing in`RELEASE`. However, you can add additional configuration to this behavior.
+
+In order to enable detailed callstack logging set `Hooks.recordCallStackOnError` flag.
+```swift
+    /* add somewhere in
+    func application(_ application: UIApplication, didFinishLaunchingWithOptions launchOptions: [UIApplicationLaunchOptionsKey : Any]? = nil)
+    */
+    Hooks.recordCallStackOnError = true
+    ...
+    Observable.just(error).subscribe(onNext: { // detailed callstack will be printed out
+        print($0)
+    })
+
+```
+
+Implement `Hooks.customCaptureSubscriptionCallstack` closure, if you need custom way of grabbing logs.
+
+Implement `Hooks.defaultErrorHandler` if you need custom reaction on unhandled errors (e.g. send stacktrace to analytics). 
+
 ## Debugging Compile Errors
 
 When writing elegant RxSwift/RxCocoa code, you are probably relying heavily on compiler to deduce types of `Observable`s. This is one of the reasons why Swift is awesome, but it can also be frustrating sometimes.

RxSwift/ObservableType+Extensions.swift

@@ -93,7 +93,10 @@ extension Hooks {
     fileprivate static var _defaultErrorHandler: DefaultErrorHandler = { subscriptionCallStack, error in
         #if DEBUG
             let serializedCallStack = subscriptionCallStack.joined(separator: "\n")
-            print("Unhandled error happened: \(error)\n subscription called from:\n\(serializedCallStack)")
+            print("Unhandled error happened: \(error)")
+            if !serializedCallStack.isEmpty {
+                print("subscription called from:\n\(serializedCallStack)")
+            }
         #endif
     }
     fileprivate static var _customCaptureSubscriptionCallstack: CustomCaptureSubscriptionCallstack = {