main: friendlier error message on wrong number of arguments

Before:
	Usage: gocryptfs [OPTIONS] CIPHERDIR MOUNTPOINT

After:
	Wrong number of arguments (have 9, want 2). You passed: "-nosyslog" "." "asd" "-q" "ß" "asdf" "fg" "gh" "sdf" "asd fs\\dfg"
	Usage: gocryptfs [OPTIONS] CIPHERDIR MOUNTPOINT

1 files changed, 10 insertions, 1 deletions

diff --git a/main.go b/main.go
index 5ae8146..2033e23 100644
--- a/main.go
+++ b/main.go
@@ -214,6 +214,15 @@ func main() {
 		}
 		changePassword(&args) // does not return
 	}
-	// Mount
+	// Default operation: mount.
+	if flagSet.NArg() != 2 {
+		prettyArgs := fmt.Sprintf("%q", os.Args[1:])
+		// Get rid of "[" and "]"
+		prettyArgs = prettyArgs[1 : len(prettyArgs)-1]
+		tlog.Info.Printf("Wrong number of arguments (have %d, want 2). You passed: %s",
+			flagSet.NArg(), prettyArgs)
+		tlog.Fatal.Printf("Usage: %s [OPTIONS] CIPHERDIR MOUNTPOINT", tlog.ProgramName)
+		os.Exit(ErrExitUsage)
+	}
 	os.Exit(doMount(&args))
 }