Improve description of error handling exercise

exercises/practice/error-handling/.docs/instructions.append.md

@@ -3,5 +3,15 @@
 ## Bash-specific instructions
 
 The goal of this exercise is to consider the number of arguments passed to your program.
-If there is exactly one argument, print a greeting message.
-Otherwise print an error message and exit with a non-zero status.
+
+Note that you can pass empty strings as arguments:
+
+`./program ""`
+
+and it is not the same as not passing the argument at all:
+
+`./program`
+
+In this excercise, if your program is run with exactly one argument (even if it is an empty string), treat is as a person's name and print a greeting message, eg. `"Hello, Kate"` (or `"Hello, "` for the empty string).
+
+Otherwise if it is run with no arguments or more than one argument, print an error message `"Usage: error_handling.sh <person>"` and exit with a non-zero status.