Better pattern matcher error message.

For the common case when someone hasn't quite grokked
the significance of lower case in a pattern match.  I'd
like to make all the unreachables errors, not warnings,
but there may be a bug or two to clear out first.

class A {
  def badEquals(x: Any, y: Any) = x match {
    case y => true
    case _ => false
  }
}

a.scala:3: warning: patterns after a variable pattern cannot match (SLS 8.1.1)
If you intended to match against parameter y of method badEquals, you must use backticks, like: case `y` =>
    case y => true
         ^
a.scala:4: warning: unreachable code due to variable pattern 'y' on line 3
    case _ => false
              ^
two warnings found

1 files changed, 5 insertions, 2 deletions

diff --git a/test/files/neg/t6048.check b/test/files/neg/t6048.check
index 051f41877e..5bdf2eca88 100644
--- a/test/files/neg/t6048.check
+++ b/test/files/neg/t6048.check
@@ -4,7 +4,10 @@ t6048.scala:3: error: unreachable code
 t6048.scala:8: error: unreachable code
     case _ if false => x // unreachable
                        ^
-t6048.scala:14: error: unreachable code
+t6048.scala:13: error: patterns after a variable pattern cannot match (SLS 8.1.1)
+    case _ => x
+         ^
+t6048.scala:14: error: unreachable code due to variable pattern on line 13
     case 5 if true  => x // unreachable
                        ^
-three errors found
+four errors found