Handle ValueError in magic %rerun

[jmsdao]

Fixes: #12948

Added a try/except block to handle non-integer arguments.

(venv) ipython$ ipython
Python 3.9.5 (default, May  5 2021, 02:58:34)
Type 'copyright', 'credits' or 'license' for more information
IPython 8.0.0.dev -- An enhanced Interactive Python. Type '?' for help.

In [1]: %rerun -l one
Argument must be an integer.

All tests pass after running iptest:

IPython version: 8.0.0.dev
IPython commit : 90c1297 (repository)
IPython package: /mnt/c/Users/James/Documents/git_projects/ipython/IPython
Python version : 3.9.5 (default, May  5 2021, 02:58:34) [GCC 7.5.0]
sys.executable : /mnt/c/Users/James/Documents/git_projects/ipython/venv/bin/python3.9
Platform       : Linux-4.4.0-19041-Microsoft-x86_64-with-glibc2.27

Tools and libraries available at test time:
   pygments

Tools and libraries NOT available at test time:
   matplotlib

Status: OK (7 test groups). Took 77.172s.

[jmsdao]

This is my first PR!

I'm still learning how to be a FOSS contributor. Please let me know of any improvements I can make on how I'm doing my PRs!
Any links to further reading is also much appreciated!

[MrMino]

Hi @jmsdao. It looks ok, you just need to darker it . Also, just so you know: normally a PR like this requires adding a whatsnew entry, but this one is small (and it's a similar situation to #12980), so it's ok.

[nicolaslazo]

Problem was addressed in commit 3abc1a3fc :D

@meeseeksdev close