Clarify how to set compiler and runtime JDKs (#29101)

This commit enhances the error messages reported when JAVA_HOME and
RUNTIME_JAVA_HOME are not correctly set to point towards the minimum
compiler and minimum runtime JDKs that are expected by the builds. The
previous error message would say:

Java 1.9 or above is required to build Elasticsearch

which is confusing if the user does have a JDK 9 installation and is
even the version that they have on their path yet they have JAVA_HOME
pointing to another JDK installation. The error message reported after
this change is:

the environment variable JAVA_HOME must be set to a JDK installation directory for Java 1.9 but is [/usr/java/jdk-8] corresponding to [1.8]

buildSrc/src/main/groovy/org/elasticsearch/gradle/BuildPlugin.groovy

@@ -140,16 +140,22 @@ class BuildPlugin implements Plugin<Project> {
 
             final GradleVersion minGradle = GradleVersion.version('4.3')
             if (currentGradleVersion < minGradle) {
-                throw new GradleException("${minGradle} or above is required to build elasticsearch")
+                throw new GradleException("${minGradle} or above is required to build Elasticsearch")
             }
 
             // enforce Java version
             if (compilerJavaVersionEnum < minimumCompilerVersion) {
-                throw new GradleException("Java ${minimumCompilerVersion} or above is required to build Elasticsearch")
+                final String message =
+                        "the environment variable JAVA_HOME must be set to a JDK installation directory for Java ${minimumCompilerVersion}" +
+                                " but is [${compilerJavaHome}] corresponding to [${compilerJavaVersionEnum}]"
+                throw new GradleException(message)
             }
 
             if (runtimeJavaVersionEnum < minimumRuntimeVersion) {
-                throw new GradleException("Java ${minimumRuntimeVersion} or above is required to run Elasticsearch")
+                final String message =
+                        "the environment variable RUNTIME_JAVA_HOME must be set to a JDK installation directory for Java ${minimumRuntimeVersion}" +
+                                " but is [${runtimeJavaHome}] corresponding to [${runtimeJavaVersionEnum}]"
+                throw new GradleException(message)
             }
 
             project.rootProject.ext.compilerJavaHome = compilerJavaHome