Ensure that NSLocationWhenInUseUsageDescription is set, throw error if not

Summary:
As per #750, throw error when trying to use the geolocation module and this key is not set. cc @frantic
Closes https://github.com/facebook/react-native/pull/762
Github Author: Brent Vatne <brent.vatne@madriska.com>

Test Plan: Imported from GitHub, without a `Test Plan:` line.

Examples/SampleApp/iOS/Info.plist

@@ -36,5 +36,7 @@
 	</array>
 	<key>UIViewControllerBasedStatusBarAppearance</key>
 	<false/>
+	<key>NSLocationWhenInUseUsageDescription</key>
+	<string></string>
 </dict>
 </plist>

Libraries/Geolocation/Geolocation.ios.js

@@ -23,11 +23,9 @@ var subscriptions = [];
 var updatesEnabled = false;
 
 /**
- * /!\ ATTENTION /!\
- * You need to add NSLocationWhenInUseUsageDescription key
- * in Info.plist to enable geolocation, otherwise it's going
- * to *fail silently*!
- * \!/           \!/
+ * You need to include the `NSLocationWhenInUseUsageDescription` key
+ * in Info.plist to enable geolocation. Geolocation is enabled by default
+ * when you create a project with `react-native init`.
  *
  * Geolocation follows the MDN specification:
  * https://developer.mozilla.org/en-US/docs/Web/API/Geolocation

Libraries/Geolocation/RCTLocationObserver.m

@@ -155,6 +155,8 @@ RCT_EXPORT_MODULE()
 
 RCT_EXPORT_METHOD(startObserving:(NSDictionary *)optionsJSON)
 {
+  [self checkLocationConfig];
+
   dispatch_async(dispatch_get_main_queue(), ^{
 
     // Select best options
@@ -189,6 +191,8 @@ RCT_EXPORT_METHOD(getCurrentPosition:(NSDictionary *)optionsJSON
                   withSuccessCallback:(RCTResponseSenderBlock)successBlock
                   errorCallback:(RCTResponseSenderBlock)errorBlock)
 {
+  [self checkLocationConfig];
+
   if (!successBlock) {
     RCTLogError(@"%@.getCurrentPosition called with nil success parameter.", [self class]);
     return;
@@ -319,4 +323,11 @@ RCT_EXPORT_METHOD(getCurrentPosition:(NSDictionary *)optionsJSON
   _locationManager.desiredAccuracy = RCT_DEFAULT_LOCATION_ACCURACY;
 }
 
+- (void)checkLocationConfig
+{
+  if (![[NSBundle mainBundle] objectForInfoDictionaryKey:@"NSLocationWhenInUseUsageDescription"]) {
+    RCTLogError(@"NSLocationWhenInUseUsageDescription key must be present in Info.plist to use geolocation.");
+  }
+}
+
 @end