…nvalid inputs

# Conflicts:
#	src/main/java/Duke.java

Add additional exceptions with better handling.

…mmand and Parser classes.

# Conflicts:
#	src/main/java/commands/VoidCommand.java

# Conflicts:
#	src/main/java/elems/TaskList.java

When TaskList methods are called, the ArrayList of Tasks should never be
null.

Adding assertions allows us to ensure that this invariant is always
true.

Let's add assertions to the TaskList methods to ensure that all TaskList
Objects which are targets for method invocation have non-null ArrayLists
at the time of method calling.

execute method in the AddCommmand class is long and can be hard to debug
 in the future. If more commands are added, the code will get even
longer.

Let's abstract out the steps taken in each case into a separate method.

Future additions of AddCommands can adopt the same style of execution.

add assertions

* 'master' of https://github.com/clydelhui/ip:
  add assertions

Improve code quality

The user may wish to sort the task list by description to make it easy to
read.

Let's add a sorting feature to enable the user to do this.

Some commands such as list and sort will access the list elements. If
the list is empty, they are unable to work as intended.

Let's add an exception to show the case when an empty list is being
accessed by commands that work with the whole list.