Provide a more helpful error message when we detect the user is tryin… (#15032)

Fixes https://github.com/flutter/flutter/issues/14950

bin/flutter

@@ -128,6 +128,9 @@ fi
 # Test if the flutter directory is a git clone (otherwise git rev-parse HEAD would fail)
 if [ ! -e "$FLUTTER_ROOT/.git" ]; then
   echo "Error: The Flutter directory is not a clone of the GitHub project."
+  echo "       The flutter tool requires Git in order to operate properly;"
+  echo "       to set up Flutter, run the following command:"
+  echo "       git clone -b beta https://github.com/flutter/flutter.git"
   exit 1
 fi
 

bin/flutter.bat

@@ -38,6 +38,9 @@ where /q git || ECHO Error: Unable to find git in your PATH. && EXIT /B 1
 REM  Test if the flutter directory is a git clone, otherwise git rev-parse HEAD would fail
 IF NOT EXIST "%flutter_root%\.git" (
   ECHO Error: The Flutter directory is not a clone of the GitHub project.
+  ECHO        The flutter tool requires Git in order to operate properly;
+  ECHO        to set up Flutter, run the following command:
+  ECHO        git clone -b beta https://github.com/flutter/flutter.git
   EXIT /B 1
 )