fail more gracefully when creating projects if adb is broken

Somehow my `adb` is broken, but still we should fail more graceously in this case:

1. remove adb from \AppData\Local\Android\sdk\platform-tools
1. try to create a Flutter project using IntelliJ:

Flutter crash report; please file at https://github.com/flutter/flutter/issues.

## command

flutter --no-color create untitled

## exception

ArgumentError: Invalid argument(s): Cannot find executable for C:\Users\mit\AppData\Local\Android\sdk\platform-tools\adb.

```
#0 _getExecutable (package:process/src/interface/local_process_manager.dart:115)
#1 LocalProcessManager.runSync (package:process/src/interface/local_process_manager.dart:86)
#2 _runWithLoggingSync (package:flutter_tools/src/base/process.dart:286)
#3 runSync (package:flutter_tools/src/base/process.dart:261)
#4 getAdbDevices (package:flutter_tools/src/android/android_device.dart:549)
#5 AndroidDevices.pollingGetDevices (package:flutter_tools/src/android/android_device.dart:51)

#6 PollingDeviceDiscovery.devices (package:flutter_tools/src/device.dart:138)

#7 DeviceManager.getAllConnectedDevices (package:flutter_tools/src/device.dart:91)

#8 DeviceValidator.validate (package:flutter_tools/src/doctor.dart:493)

#9 Doctor.diagnose (package:flutter_tools/src/doctor.dart:105)

#10 CreateCommand.runCommand (package:flutter_tools/src/commands/create.dart:224)

#11 FlutterCommand.verifyThenRunCommand (package:flutter_tools/src/runner/flutter_command.dart:210)

#12 FlutterCommand.run (package:flutter_tools/src/runner/flutter_command.dart:156)

#13 CommandRunner.runCommand (package:args/command_runner.dart:194)

#14 FlutterCommandRunner.runCommand (package:flutter_tools/src/runner/flutter_command_runner.dart:277)

#15 CommandRunner.run. (package:args/command_runner.dart:109)
#16 new Future.sync (dart:async/future.dart:206)
#17 CommandRunner.run (package:args/command_runner.dart:109)
#18 FlutterCommandRunner.run (package:flutter_tools/src/runner/flutter_command_runner.dart:161)
#19 run. (package:flutter_tools/executable.dart:142)

#20 AppContext._run (package:flutter_tools/src/base/context.dart:76)

#21 AppContext.runInZone. (package:flutter_tools/src/base/context.dart:66)
#22 _rootRun (dart:async/zone.dart:1120)
#23 _CustomZone.run (dart:async/zone.dart:1001)
#24 runZoned (dart:async/zone.dart:1467)
#25 AppContext.runInZone (package:flutter_tools/src/base/context.dart:65)
#26 run (package:flutter_tools/executable.dart:114)

#27 main (package:flutter_tools/executable.dart:67)

#28 main (file:///C:/Users/mit/dev/github/flutter/packages/flutter_tools/bin/flutter_tools.dart:16)
#29 _startIsolate. (dart:isolate-patch/isolate_patch.dart:263)
#30 _RawReceivePortImpl._handleMessage (dart:isolate-patch/isolate_patch.dart:151)
```

## flutter doctor

```
encountered exception: Invalid argument(s): Cannot find executable for C:\Users\mit\AppData\Local\Android\sdk\platform-tools\adb.

#0 _getExecutable (package:process/src/interface/local_process_manager.dart:115)
#1 LocalProcessManager.runSync (package:process/src/interface/local_process_manager.dart:86)
#2 _runWithLoggingSync (package:flutter_tools/src/base/process.dart:286)
#3 runSync (package:flutter_tools/src/base/process.dart:261)
#4 getAdbDevices (package:flutter_tools/src/android/android_device.dart:549)
#5 AndroidDevices.pollingGetDevices (package:flutter_tools/src/android/android_device.dart:51)

#6 PollingDeviceDiscovery.devices (package:flutter_tools/src/device.dart:138)

#7 DeviceManager.getAllConnectedDevices (package:flutter_tools/src/device.dart:91)

#8 DeviceValidator.validate (package:flutter_tools/src/doctor.dart:493)

#9 Doctor.diagnose (package:flutter_tools/src/doctor.dart:105)

#10 _doctorText. (package:flutter_tools/executable.dart:289)
#11 AppContext._run (package:flutter_tools/src/base/context.dart:76)

#12 AppContext.runInZone. (package:flutter_tools/src/base/context.dart:66)
#13 _rootRun (dart:async/zone.dart:1120)
#14 _CustomZone.run (dart:async/zone.dart:1001)
#15 runZoned (dart:async/zone.dart:1467)
#16 AppContext.runInZone (package:flutter_tools/src/base/context.dart:65)
#17 _doctorText (package:flutter_tools/executable.dart:289)

#18 _createLocalCrashReport (package:flutter_tools/executable.dart:264)

#19 _handleToolError (package:flutter_tools/executable.dart:219)

#20 run. (package:flutter_tools/executable.dart:146)

#21 AppContext._run (package:flutter_tools/src/base/context.dart:76)

#22 AppContext.runInZone. (package:flutter_tools/src/base/context.dart:66)
#23 _rootRun (dart:async/zone.dart:1120)
#24 _CustomZone.run (dart:async/zone.dart:1001)
#25 runZoned (dart:async/zone.dart:1467)
#26 AppContext.runInZone (package:flutter_tools/src/base/context.dart:65)
#27 run (package:flutter_tools/executable.dart:114)

#28 main (package:flutter_tools/executable.dart:67)

#29 main (file:///C:/Users/mit/dev/github/flutter/packages/flutter_tools/bin/flutter_tools.dart:16)
#30 _startIsolate. (dart:isolate-patch/isolate_patch.dart:263)
#31 _RawReceivePortImpl._handleMessage (dart:isolate-patch/isolate_patch.dart:151)
```