Improve libimobiledevice doctor messages (#12633)

The libimobiledevice suite of tools do not include version information.
A simple way to verify they meet our version requirements is to run
idevice_id -l, which will fail when older versions are run against iOS
devices with newer versions of iOS installed.

Unfortunately, idevice_id -l will also fail when libimobiledevice is up
to date, but the attached devices have never been paired with the host
machine in Xcode.

This patch updates the error message to help guide the user in such
situations.

packages/flutter_tools/lib/src/ios/ios_workflow.dart

@@ -135,7 +135,9 @@ class IOSWorkflow extends DoctorValidator implements Workflow {
       } else if (!await iMobileDevice.isWorking) {
         brewStatus = ValidationType.partial;
         messages.add(new ValidationMessage.error(
-            'libimobiledevice and ideviceinstaller may require updating. To update, run:\n'
+            'Verify that all connected devices have been paired with this computer in Xcode.\n'
+            'If all devices have been paired, libimobiledevice and ideviceinstaller may require updating.\n'
+            'To update, run:\n'
             '  brew uninstall --ignore-dependencies libimobiledevice\n'
             '  brew install --HEAD libimobiledevice\n'
             '  brew install ideviceinstaller'
@@ -143,7 +145,7 @@ class IOSWorkflow extends DoctorValidator implements Workflow {
       } else if (!await hasIDeviceInstaller) {
         brewStatus = ValidationType.partial;
         messages.add(new ValidationMessage.error(
-          'ideviceinstaller not available; this is used to discover connected iOS devices.\n'
+          'ideviceinstaller is not installed; this is used to discover connected iOS devices.\n'
           'To install, run:\n'
           '  brew install --HEAD libimobiledevice\n'
           '  brew install ideviceinstaller'