Fix building ProviderNotRegistered exception message (#1020)

Author: norkunas

ProviderAggregator.php

@@ -145,7 +145,7 @@ public function registerProviders(array $providers = []): self
     public function using(string $name): self
     {
         if (!isset($this->providers[$name])) {
-            throw ProviderNotRegistered::create($name ?? '', $this->providers);
+            throw ProviderNotRegistered::create($name, array_keys($this->providers));
         }
 
         $this->provider = $this->providers[$name];

Tests/ProviderAggregatorTest.php

@@ -85,9 +85,12 @@ public function testRegisterProviders()
 
     /**
      * @expectedException \Geocoder\Exception\ProviderNotRegistered
+     * @expectedExceptionMessage Provider "non_existant" is not registered, so you cannot use it. Did you forget to register it or made a typo? Registered providers are: test1.
      */
     public function testUsingNonExistantProviderShouldThrowAnException()
     {
+        $this->geocoder->registerProvider(new MockProvider('test1'));
+
         $this->geocoder->using('non_existant');
     }