Check that env var KAITAI_STRUCT_VERSION matches build.sbt VERSION

See 5340ff3#commitcomment-99529992
- asserting that the versions match seems to be the easiest way to avoid
that problem.

Throwing `MessageOnlyException` to indicate the error is inspired by
https://github.com/sbt/sbt-dynver/tree/125101ba#sanity-checking-the-version.

build.sbt

@@ -23,7 +23,21 @@ lazy val compiler = crossProject.in(file(".")).
   enablePlugins(JavaAppPackaging).
   settings(
     organization := "io.kaitai",
-    version := sys.env.getOrElse("KAITAI_STRUCT_VERSION", VERSION),
+    version := {
+      sys.env.get("KAITAI_STRUCT_VERSION") match {
+        case Some(ver) =>
+          if (VERSION.endsWith("-SNAPSHOT")) {
+            if (!ver.startsWith(VERSION))
+              throw new MessageOnlyException(s"Environment variable KAITAI_STRUCT_VERSION '$ver' doesn't start with build.sbt VERSION '$VERSION'")
+          } else {
+            if (ver != VERSION)
+              throw new MessageOnlyException(s"Environment variable KAITAI_STRUCT_VERSION '$ver' is not equal to build.sbt VERSION '$VERSION'")
+          }
+          ver
+        case None =>
+          VERSION
+      }
+    },
     licenses := Seq(("GPL-3.0", url("https://opensource.org/licenses/GPL-3.0"))),
     scalaVersion := "2.12.12",