Surround iex.exs with a try..catch for safety

[fhunleth]

Make it easier for people to safely modify their iex.exs by giving them
a starter try..catch block to put stuff in.

[mobileoverlord]

Mix already protects from this, but I would have to look to see when it was added. In the following example I set true = false and upload it to the board.

Interactive Elixir (1.10.2) - press Ctrl+C to exit (type h() ENTER for help)
Toolshed imported. Run h(Toolshed) for more info
RingLogger is collecting log messages from Elixir and Linux. To see the
messages, either attach the current IEx session to the logger:

  RingLogger.attach

or print the next messages in the log:

  RingLogger.next

Error while evaluating: /etc/iex.exs
** (MatchError) no match of right hand side value: false

iex(1)> 

[fhunleth]

Cool. It worked for me too.

That's way better that it was added upstream. Here's the PR: elixir-lang/elixir#8619.