Add documentation for FailureAnalyzer

Closes gh-6775

Author: snicoll

spring-boot-docs/src/main/asciidoc/howto.adoc

@@ -22,6 +22,30 @@ can send us a {github-code}[pull request].
 == Spring Boot application
 
 
+[[howto-failure-analyzer]]
+=== Create your own FailureAnalyzer
+{dc-spring-boot}/diagnostics/FailureAnalyzer.{dc-ext}[[`FailureAnalyzer`] is a great way
+to intercept an exception on startup and turn it into a human-readable message, wrapped
+into a {dc-spring-boot}/diagnostics/FailureAnalysis.{dc-ext}[[`FailureAnalysis`]. Spring
+Boot provides such analyzer for application context related exceptions, JSR-303
+validations and more. It is actually very easy to create your own.
+
+`AbstractFailureAnalyzer` is a convenient extension of `FailureAnalyzer` that checks the
+presence of a specified exception type in the exception to handle. You can extend from
+that so that your implementation gets a chance to handle the exception only when it is
+actually present. If for whatever reason you can't handle the exception, return `null`
+to let another implementation a chance to handle the exception.
+
+`FailureAnalyzer` implementations are to be registered in a `META-INF/spring.factories`:
+the following registers `ProjectConstraintViolationFailureAnalyzer`:
+
+[source,properties,indent=0]
+----
+	org.springframework.boot.diagnostics.FailureAnalyzer=\
+	com.example.ProjectConstraintViolationFailureAnalyzer
+----
+
+
 
 [[howto-troubleshoot-auto-configuration]]
 === Troubleshoot auto-configuration

spring-boot-docs/src/main/asciidoc/spring-boot-features.adoc

@@ -46,6 +46,29 @@ When your application starts you should see something similar to the following:
 By default `INFO` logging messages will be shown, including some relevant startup details
 such as the user that launched the application.
 
+If your application fails to start, registered `FailureAnalyzers` get a chance to provide
+a dedicated error message and a concrete action to fix the problem. For instance if you
+start a web application on port `8080` and that port is already in use, you should see
+something similar to the following:
+
+[indent=0]
+----
+***************************
+APPLICATION FAILED TO START
+***************************
+
+Description:
+
+Embedded servlet container failed to start. Port 8080 was already in use.
+
+Action:
+
+Identify and stop the process that's listening on port 8080 or configure this application to listen on another port.
+----
+
+NOTE: Spring Boot provides numerous `FailureAnalyzer` implementations and you can
+<<howto.adoc#howto-failure-analyzer,add your own>> very easily.
+
 
 [[boot-features-banner]]
 === Customizing the Banner