Suggestion to fix the error when "Apple bundle requires an Apple platform" is shown
Suggestion to fix the error when "Apple bundle requires an Apple platform" is shown

Summary:
The message is hard to understand and people don't know how to fix it.
We're adding a message to help the user.

Test Plan:
```
$ buck-beta build --config cxx.default_platform=iphoneosarm-arm64 [app]
::: Faking buck version e7a46faf1c81e21c846b1b7d1d1a29194281e0e0, despite your buck directory not being that version.
::: Faking buck version e7a46faf1c81e21c846b1b7d1d1a29194281e0e0, despite your buck directory not being that version.
::: Faking buck version e7a46faf1c81e21c846b1b7d1d1a29194281e0e0, despite your buck directory not being that version.
[-] PROCESSING BUCK FILES...FINISHED 5.5s [100%] 🐳  New buck daemon
[-] DOWNLOADING... (0.00 B/S AVG, TOTAL: 0.00 B, 0 Artifacts)
[-] BUILDING...FINISHED 3.8s
    Details: http://localhost:62360/trace/6ee155e7-465b-462a-8f86-69f7a2eeeaff
BUILD FAILED: ...#default,dwarf,no-include-frameworks: Apple bundle requires an Apple platform, found 'default'

A common cause of this error is that the required SDK is missing.
Please check whether it's installed and retry.
```

Reviewed By: ttsugriy

fbshipit-source-id: ab3f400

rmaz/buck,ilya-klyuchnikov/buck,rmaz/buck,ilya-klyuchnikov/buck,zpao/buck,shs96c/buck,shs96c/buck,LegNeato/buck,robbertvanginkel/buck,romanoid/buck,robbertvanginkel/buck,shs96c/buck,zhan-xiong/buck,brettwooldridge/buck,JoelMarcey/buck,JoelMarcey/buck,romanoid/buck,facebook/buck,shybovycha/buck,nguyentruongtho/buck,shybovycha/buck,romanoid/buck,robbertvanginkel/buck,JoelMarcey/buck,clonetwin26/buck,zpao/buck,SeleniumHQ/buck,Addepar/buck,LegNeato/buck,romanoid/buck,ilya-klyuchnikov/buck,brettwooldridge/buck,ilya-klyuchnikov/buck,rmaz/buck,robbertvanginkel/buck,dsyang/buck,ilya-klyuchnikov/buck,zpao/buck,dsyang/buck,SeleniumHQ/buck,zhan-xiong/buck,shybovycha/buck,dsyang/buck,zhan-xiong/buck,SeleniumHQ/buck,zhan-xiong/buck,brettwooldridge/buck,JoelMarcey/buck,kageiit/buck,clonetwin26/buck,SeleniumHQ/buck,zpao/buck,brettwooldridge/buck,clonetwin26/buck,romanoid/buck,ilya-klyuchnikov/buck,LegNeato/buck,Addepar/buck,nguyentruongtho/buck,robbertvanginkel/buck,SeleniumHQ/buck,rmaz/buck,k21/buck,k21/buck,JoelMarcey/buck,shybovycha/buck,rmaz/buck,LegNeato/buck,kageiit/buck,zhan-xiong/buck,shybovycha/buck,shs96c/buck,clonetwin26/buck,zpao/buck,dsyang/buck,rmaz/buck,romanoid/buck,brettwooldridge/buck,LegNeato/buck,Addepar/buck,SeleniumHQ/buck,romanoid/buck,nguyentruongtho/buck,clonetwin26/buck,SeleniumHQ/buck,facebook/buck,zhan-xiong/buck,ilya-klyuchnikov/buck,robbertvanginkel/buck,ilya-klyuchnikov/buck,clonetwin26/buck,LegNeato/buck,LegNeato/buck,k21/buck,rmaz/buck,romanoid/buck,LegNeato/buck,zhan-xiong/buck,romanoid/buck,clonetwin26/buck,k21/buck,shs96c/buck,brettwooldridge/buck,zhan-xiong/buck,brettwooldridge/buck,brettwooldridge/buck,k21/buck,robbertvanginkel/buck,ilya-klyuchnikov/buck,shybovycha/buck,facebook/buck,JoelMarcey/buck,rmaz/buck,clonetwin26/buck,kageiit/buck,rmaz/buck,zhan-xiong/buck,ilya-klyuchnikov/buck,Addepar/buck,k21/buck,brettwooldridge/buck,romanoid/buck,Addepar/buck,shybovycha/buck,kageiit/buck,kageiit/buck,Addepar/buck,brettwooldridge/buck,dsyang/buck,SeleniumHQ/buck,shybovycha/buck,Addepar/buck,robbertvanginkel/buck,nguyentruongtho/buck,robbertvanginkel/buck,dsyang/buck,facebook/buck,brettwooldridge/buck,SeleniumHQ/buck,shs96c/buck,LegNeato/buck,zpao/buck,k21/buck,shs96c/buck,clonetwin26/buck,zhan-xiong/buck,nguyentruongtho/buck,SeleniumHQ/buck,shs96c/buck,shs96c/buck,zhan-xiong/buck,facebook/buck,rmaz/buck,shybovycha/buck,k21/buck,dsyang/buck,dsyang/buck,SeleniumHQ/buck,kageiit/buck,JoelMarcey/buck,zhan-xiong/buck,nguyentruongtho/buck,robbertvanginkel/buck,JoelMarcey/buck,Addepar/buck,ilya-klyuchnikov/buck,nguyentruongtho/buck,k21/buck,dsyang/buck,LegNeato/buck,zhan-xiong/buck,SeleniumHQ/buck,clonetwin26/buck,Addepar/buck,JoelMarcey/buck,Addepar/buck,rmaz/buck,rmaz/buck,robbertvanginkel/buck,ilya-klyuchnikov/buck,shybovycha/buck,k21/buck,dsyang/buck,romanoid/buck,LegNeato/buck,LegNeato/buck,Addepar/buck,shybovycha/buck,romanoid/buck,zpao/buck,JoelMarcey/buck,SeleniumHQ/buck,shs96c/buck,shs96c/buck,shs96c/buck,LegNeato/buck,Addepar/buck,clonetwin26/buck,dsyang/buck,k21/buck,brettwooldridge/buck,k21/buck,JoelMarcey/buck,romanoid/buck,dsyang/buck,kageiit/buck,k21/buck,facebook/buck,clonetwin26/buck,JoelMarcey/buck,Addepar/buck,robbertvanginkel/buck,robbertvanginkel/buck,shs96c/buck,shybovycha/buck,rmaz/buck,shybovycha/buck,clonetwin26/buck,dsyang/buck,facebook/buck,brettwooldridge/buck,ilya-klyuchnikov/buck,JoelMarcey/buck