Use custom exception handler to prettify the interactive output

Because of limitations in both eval() and Boris, we need to
create a custom exception handler that simply echos out any
errors caused by command line functions.

Using ShellWrap in regular apps will use whichever method defined
which can show a stacktrace if you like.

# Please enter the commit message for your changes. Lines starting
# with '#' will be ignored, and an empty message aborts the commit.
# On branch master
# Changes to be committed:
#   (use "git reset HEAD <file>..." to unstage)
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#	modified:   README.md
#	modified:   bin/shellwrap
#	modified:   examples/ls.php
#	modified:   src/MrRio/ShellWrap.php
#	new file:   src/MrRio/ShellWrapException.php
#	new file:   thingy
#