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, 1 insertions, 0 deletions

diff --git a/test/files/neg/newpat_unreachable.flags b/test/files/neg/newpat_unreachable.flags
new file mode 100644
index 0000000000..85d8eb2ba2
--- /dev/null
+++ b/test/files/neg/newpat_unreachable.flags
@@ -0,0 +1 @@
+-Xfatal-warnings