Hotfix for SBT plugin

SBT plugin attempts to obtain the version of the latest
nightly build from the Dotty website. Sometimes, on release,
this version is set on the website before the actual nightly
publish takes place, which leads to an exception. This commit is
a hotfix instructing the plugin to fall back to the latest published
version of Dotty.

Author: anatoliykmetyuk

sbt-dotty/src/dotty/tools/sbtplugin/DottyPlugin.scala

@@ -28,12 +28,18 @@ object DottyPlugin extends AutoPlugin {
       val nightly = try {
         // get majorVersion from dotty.epfl.ch
         val source0 = Source.fromURL("http://dotty.epfl.ch/versions/latest-nightly-base")
-        val majorVersion = source0.getLines().toSeq.head
+        val majorVersionFromWebsite = source0.getLines().toSeq.head
         source0.close()
 
         // get latest nightly version from maven
-        val source1 =
-          Source.fromURL(s"http://repo1.maven.org/maven2/ch/epfl/lamp/dotty_$majorVersion/maven-metadata.xml")
+        def fetchSource(version: String): (scala.io.BufferedSource, String) =
+          try Source.fromURL(s"http://repo1.maven.org/maven2/ch/epfl/lamp/dotty_$version/maven-metadata.xml") -> version
+          catch { case t: java.io.FileNotFoundException =>
+            val major :: minor :: Nil = version.split(".").toList
+            if (minor.toInt <= 0) throw t
+            else fetchSource(s"$major.${minor.toInt - 1}")
+          }
+        val (source1, majorVersion) = fetchSource(majorVersionFromWebsite)
         val Version = s"      <version>($majorVersion\\..*-bin.*)</version>".r
         val nightly = source1
           .getLines()