Provide a more helpful error message if files support fails due to exec() being disabled.

docs/exceptions/filesExecNotAvailable.md

@@ -0,0 +1,51 @@
+FilesFinder requires that exec() is available.
+----------------------------------------------
+
+Example PHP:
+```
+<?php
+
+require_once __DIR__ . '/../vendor/autoload.php';
+
+use HaydenPierce\ClassFinder\ClassFinder;
+
+ClassFinder::enableFilesSupport();
+$classes = ClassFinder::getClassesInNamespace('Acme\Foo\Bar');
+```
+
+Results in this exception:
+
+> FilesFinder requires that exec() is available. Check your php.ini to see if it is disabled.
+
+When running ClassFinder with support for autoloaded classes in `files`, ClassFinder must execute the included file in a
+shell to determine any defined classes in it. `exec()` is used to accomplish this. In some environments, hosts may 
+intentionally disable the use `exec()` as a security or performance precaution. 
+
+Possible Solution 1
+-------------------
+
+Disable `files` support.
+
+The majority of users won't need `files` support. If you've copy / pasted a snippet (including from documentation here) 
+that enabled it, you should remove it and see if you're part of the 99% that doesn't need this feature. You may also
+want to explicitly disable it:
+
+```
+ClassFinder::disableFilesSupport();
+$classes = ClassFinder::getClassesInNamespace('Acme\Foo\Bar');
+```
+
+Possible Solution 2
+-------------------
+
+Ensure `exec()` is available to PHP.
+
+Find your `php.ini` file and look for a configuration value called `disabled_functions`:
+
+```
+disable_functions = exec,passthru,shell_exec,system,proc_open,popen
+```
+
+Remove exec from the list and restart your webserver.
+
+

src/Files/FilesFinder.php

@@ -1,15 +1,28 @@
 <?php
 namespace HaydenPierce\ClassFinder\Files;
 
+use HaydenPierce\ClassFinder\Exception\ClassFinderException;
 use HaydenPierce\ClassFinder\FinderInterface;
 
 class FilesFinder implements FinderInterface
 {
     private $factory;
 
+    /**
+     * FilesFinder constructor.
+     * @param FilesEntryFactory $factory
+     * @throws ClassFinderException
+     */
     public function __construct(FilesEntryFactory $factory)
     {
         $this->factory = $factory;
+
+        if (!function_exists('exec')) {
+            throw new ClassFinderException(sprintf(
+                'FilesFinder requires that exec() is available. Check your php.ini to see if it is disabled. See "%s" for details.',
+                'https://gitlab.com/hpierce1102/ClassFinder/blob/master/docs/exceptions/filesExecNotAvailable.md'
+            ));
+        }
     }
 
     public function isNamespaceKnown($namespace)