Update SPI Error Messages (#16619)

* Updated HttpClientProviders error message

* Updated JsonSerializerProviders error message

* Update links to aka.ms links

sdk/core/azure-core/src/main/java/com/azure/core/implementation/http/HttpClientProviders.java

@@ -13,8 +13,11 @@
  */
 public final class HttpClientProviders {
     private static HttpClientProvider defaultProvider;
-    private static final String CANNOT_FIND_HTTP_CLIENT =
-        "Cannot find any HttpClient provider on the classpath - unable to create a default HttpClient instance";
+    private static final String CANNOT_FIND_HTTP_CLIENT = "A request was made to load the default HttpClient provider "
+        + "but one could not be found on the classpath. If you are using a dependency manager, consider including a "
+        + "dependency on azure-core-http-netty or azure-core-http-okhttp. Depending on your existing dependencies, you "
+        + "have the choice of Netty or OkHttp implementations. Additionally, refer to "
+        + "https://aka.ms/azsdk/java/docs/custom-httpclient to learn about writing your own implementation.";
 
     static {
         ServiceLoader<HttpClientProvider> serviceLoader = ServiceLoader.load(HttpClientProvider.class);

sdk/core/azure-core/src/main/java/com/azure/core/util/serializer/JsonSerializerProviders.java

@@ -10,8 +10,12 @@
  * This class is a proxy for using a {@link JsonSerializerProvider} loaded from the classpath.
  */
 public final class JsonSerializerProviders {
-    private static final String CANNOT_FIND_JSON_SERIALIZER_PROVIDER =
-        "Cannot find any JSON serializer provider on the classpath.";
+    private static final String CANNOT_FIND_JSON_SERIALIZER_PROVIDER = "A request was made to load the default JSON "
+        + "serializer provider but one could not be found on the classpath. If you are using a dependency manager, "
+        + "consider including a dependency on azure-core-serializer-json-jackson or azure-core-serializer-json-gson. "
+        + "Depending on your existing dependencies, you have the choice of Jackson or GSON implementations. "
+        + "Additionally, refer to https://aka.ms/azsdk/java/docs/custom-jsonserializer to learn about writing your own "
+        + "implementation.";
 
     private static JsonSerializerProvider defaultProvider;
     private static boolean attemptedLoad;