RequirementsException gives foreach error when single requirement present

Could you add a test demonstrating the failure?

Exactly what scenario triggered it? Would it be better to fix the thrower that didn't properly form the array?

Looks good except for two things:

1. +++ b/core/modules/migrate/src/Exception/RequirementsException.php
   @@ -60,8 +60,13 @@ public function getRequirements() {
   +      if (is_array($requirements)) {
   +        foreach ($requirements as $value) {
   +          $output .= "$requirement_type: $value. ";
   +        }
   +      }
   +      else {
   +        $output .= "$requirement_type: $requirements. ";
   

   Code repetition makes me scrunch up my face. Maybe do something like this:

   if (is_scalar($requirements)) {
     $requirements = array($requirements);
   }
   

2. +++ b/core/modules/migrate/tests/src/Unit/Exception/RequirementsExceptionTest.php
   @@ -0,0 +1,55 @@
   +  protected $missing_requirements = ['random_jackson_pivot', '51_Eridani_b'];
   

   Should use camelCase.

+++ b/core/modules/migrate/tests/src/Unit/Exception/RequirementsExceptionTest.php
@@ -0,0 +1,55 @@
+   * Provides a list of requirements to test

Nit: missing a period.

One more.

+++ b/core/modules/migrate/tests/src/Unit/Exception/RequirementsExceptionTest.php
@@ -0,0 +1,55 @@
+  public function getRequirements() {

The core convention is to use the word "provider" in data provider names. So maybe getRequirementsProvider() is a better name for this method.

+++ b/core/modules/migrate/src/Exception/RequirementsException.php
@@ -60,6 +60,10 @@ public function getRequirements() {
+      if(!is_array($requirements)) {
+        $requirements = array($requirements);
+      }

Tiny annoying code style nit: that should be if (!is_array($requirements)). Can be fixed on commit, though.

This has passed DrupalCI and otherwise looks fine to me. RTBC!

Fixed that, and...

Committed and pushed to 8.0.x. Thanks!

Although those random test names are truly... random. :)