sttp simple could offer a better error message on jdk < 11

[lbialy]

Related to toolkit issue: scala/toolkit#39

This is basically an improvement proposal for ClassNotFoundException thrown when built-in java http client class is missing on jdks below 11.

[adamw]

Hm the exception comes from the class loader which loads sttp's classes (at least as far as I can reproduce), so I don't think we can do much about it:

Exception in thread "main" java.lang.UnsupportedClassVersionError: sttp/client4/quick$ has been compiled by a more recent version of the Java Runtime (class file version 55.0), this version of the Java Runtime only recognizes class file versions up to 52.0
	at java.lang.ClassLoader.defineClass1(Native Method)
	at java.lang.ClassLoader.defineClass(ClassLoader.java:757)
	at java.security.SecureClassLoader.defineClass(SecureClassLoader.java:142)
	at java.net.URLClassLoader.defineClass(URLClassLoader.java:473)
	at java.net.URLClassLoader.access$100(URLClassLoader.java:74)
	at java.net.URLClassLoader$1.run(URLClassLoader.java:369)
	at java.net.URLClassLoader$1.run(URLClassLoader.java:363)
	at java.security.AccessController.doPrivileged(Native Method)
	at java.net.URLClassLoader.findClass(URLClassLoader.java:362)
	at java.lang.ClassLoader.loadClass(ClassLoader.java:419)
	at sun.misc.Launcher$AppClassLoader.loadClass(Launcher.java:352)
	at java.lang.ClassLoader.loadClass(ClassLoader.java:352)
	at x$package$.test(x.scala:6)
	at test.main(x.scala:5)