[lang] Make the JDK and Xtext validation messages more clear.

Signed-off-by: Stéphane Galland <galland@arakhne.org>

plugins/io.sarl.lang/src/io/sarl/lang/validation/messages.properties

@@ -1,5 +1,5 @@
-SARLValidator_0=Couldn''t find a JDK {0} or higher on the project's classpath.
-SARLValidator_1=Couldn''t find the mandatory library ''org.eclipse.xtext.xbase.lib'' {0} or higher on the project''s classpath.
+SARLValidator_0=Couldn''t find a valid version of the JDK. Please use the JDK {0} or higher.
+SARLValidator_1=Couldn''t find a valid version of the Xtext base library in the classpath. You should add ''org.eclipse.xtext.xbase.lib'', version {0} or higher.
 SARLValidator_2=Duplicate definition of the type ''{0}''
 SARLValidator_3=A default value cannot be declared for the variadic formal parameter ''{0}''.
 SARLValidator_4=Type mismatch: cannot convert from {0} to {1}