Also check existence of shasum

hooks/pre-commit

@@ -78,6 +78,15 @@ else
 	exit 1
 fi
 
+printf "${PREFIX} Checking for shasum ... "
+command -v shasum &>/dev/null
+if [ $? == 0 ]; then
+	printf "${SUCCESS}\n"
+else
+	printf "${FAILURE}\n"
+	exit 1
+fi
+
 # Just check that running rustfmt doesn't do anything to the file. I do this instead of
 # modifying the file because I don't want to mess with the developer's index, which may
 # not only contain discrete files.
@@ -103,6 +112,4 @@ else
 	printf "${SUCCESS}\n"
 fi
 
-if [ ${FAILED} != 0 ]; then
-	exit 1
-fi
+exit ${FAILED}