Fix style.sh to check for mint availability (#11070)

Author: wu-hui

scripts/style.sh

@@ -24,6 +24,9 @@
 # Commonly
 # ./scripts/style.sh master
 
+# Fail the script if any command fails
+set -e
+
 # Strip the clang-format version output down to the major version. Examples:
 #   clang-format version 7.0.0 (tags/google/stable/2018-01-11)
 #   clang-format version google3-trunk (trunk r333779)
@@ -63,6 +66,11 @@ esac
 # /usr/local.
 export MINT_PATH=Mint
 
+if ! which mint >/dev/null 2>&1; then
+  echo "mint is not available, install with 'brew install mint'"
+  exit 1
+fi
+
 system=$(uname -s)
 
 # Joins the given arguments with the separator given as the first argument.