Parse javac non-compile-errors in JavaErrorParser

Upon startup, javac may report errors because (for instance) because it
received incorrect flags. These errors were not correctly parsed by the
JavaErrorParser and were never reported to the user.

This commit fixes this problem by adding a new parsing rule in
JavaErrorParser: errors that start with the prefix "javac:" are now
correctly parsed and reported to the user.

Fixes sbt/sbt#2256

compile/src/main/scala/sbt/compiler/javac/JavaErrorParser.scala

@@ -44,6 +44,7 @@ class JavaErrorParser(relativeDir: File = new File(new File(".").getAbsolutePath
 
   override val skipWhitespace = false
 
+  val JAVAC: Parser[String] = literal("javac")
   val CHARAT: Parser[String] = literal("^")
   val SEMICOLON: Parser[String] = literal(":") | literal("\uff1a")
   val SYMBOL: Parser[String] = allUntilChar(':') // We ignore whether it actually says "symbol" for i18n
@@ -161,8 +162,16 @@ class JavaErrorParser(relativeDir: File = new File(new File(".").getAbsolutePath
         msg
       )
     }
+  val javacError: Parser[Problem] =
+    JAVAC ~ SEMICOLON ~ restOfLine ^^ {
+      case _ ~ _ ~ error =>
+        new JavaProblem(
+          JavaNoPosition,
+          Severity.Error,
+          s"javac:$error")
+    }
 
-  val potentialProblem: Parser[Problem] = warningMessage | errorMessage | noteMessage
+  val potentialProblem: Parser[Problem] = warningMessage | errorMessage | noteMessage | javacError
 
   val javacOutput: Parser[Seq[Problem]] = rep(potentialProblem)
   /**

compile/src/test/scala/sbt/compiler/javac/javaErrorParserSpec.scala

@@ -15,6 +15,7 @@ object JavaErrorParserSpec extends Specification {
      be able to parse linux errors    $parseSampleLinux
      be able to parse windows file names $parseWindowsFile
      be able to parse windows errors  $parseSampleWindows
+     be able to parse javac errors $parseSampleJavac
   """
 
   def parseSampleLinux = {
@@ -45,6 +46,15 @@ object JavaErrorParserSpec extends Specification {
     }
   }
 
+  def parseSampleJavac = {
+    val parser = new JavaErrorParser()
+    val logger = Logger.Null
+    val problems = parser.parseProblems(sampleJavacMessage, logger)
+    def rightSize = problems must haveSize(1)
+    def rightError = problems(0).message must beEqualTo(sampleJavacMessage)
+    rightSize and rightError
+  }
+
   def sampleLinuxMessage =
     """
       |/home/me/projects/sample/src/main/Test.java:4: cannot find symbol
@@ -63,4 +73,6 @@ object JavaErrorParserSpec extends Specification {
 
   def windowsFile = """C:\Projects\sample\src\main\java\Test.java"""
   def windowsFileAndLine = s"""$windowsFile:4"""
+
+  def sampleJavacMessage = "javac: invalid flag: -foobar"
 }