Prevent showing errors for command check

[eval]

This PR gets rid of errors being shown on install when commands are missing (ie aspell missing):

$ curl -L https://github.com/bbatsov/prelude/raw/master/utils/installer.sh | sh
  % Total    % Received % Xferd  Average Speed   Time    Time     Time  Current
                                 Dload  Upload   Total   Spent    Left  Speed
  0     0    0     0    0     0      0      0 --:--:-- --:--:-- --:--:--     0
100  8206  100  8206    0     0  15613      0 --:--:-- --:--:-- --:--:--     0
 Checking to see if git is installed...  found.
 Checking to see if aspell is installed...  not found. Install aspell to benefit from flyspell-mode!
sh: line 169: hash: aspell: not found
 Backing up the existing config to /Users/gertgoet/.emacs.d.pre-prelude.tar.
 Cloning Emacs Prelude's GitHub repository...
Cloning into '/Users/gertgoet/.emacs.d'...
remote: Enumerating objects: 7457, done.
remote: Counting objects: 100% (106/106), done.
remote: Compressing objects: 100% (63/63), done.
remote: Total 7457 (delta 63), reused 45 (delta 43), pack-reused 7351 (from 3)
Receiving objects: 100% (7457/7457), 5.28 MiB | 5.48 MiB/s, done.
Resolving deltas: 100% (4528/4528), done.
 Creating the required directories.
 Byte-compiling Prelude...

  ____           _           _
 |  _ \ _ __ ___| |_   _  __| | ___
 | |_) |  __/ _ \ | | | |/ _  |/ _ \
 |  __/| | |  __/ | |_| | (_| |  __/
 |_|   |_|  \___|_|\__,_|\__,_|\___|

 ... is now installed and ready to do thy bidding, Master gertgoet!
 Don't forget to adjust the modules you want to use in /Users/gertgoet/.emacs.d/personal/prelude-modules.el!
sh: line 169: hash: aspell: not found

Because of the (repeated) error on the last line, I got the impression it stopped halfway (which was not the case).

---

Before submitting the PR make sure the following things have been done (and denote this by checking the relevant checkboxes):

• [X] The commits are consistent with our contribution guidelines
• [ ] ~~You've updated the changelog (if adding/changing user-visible functionality)~~
• [ ] ~~You've updated the user manual (if adding/changing user-visible functionality like modules, commands, configuration options, etc)~~
• [X] verified fixed using install from fork

Thanks!