Cleanup of crash output.

I finally ran across the assert (or one of them) which produces such
voluminous output that it scares the cattle. When you see "decls" in an
assert, run.

After silencing that, I extended the journey into quieting the default crash
message down, way down. I assume this will be met only with sighs of relief.
And then most challenging of all for some reason, I convinced it to only print
the information once.

Volume of crash data generated when reproducing the fsc bug shown below:

  Before this commit  1158 lines
  After this commit    27 lines

Pretty sure there's more information in the 27 lines. To reproduce the crash:

  fsc src/library/scala/collection/immutable/IntMap.scala
 fsc src/library/scala/collection/immutable/IntMap.scala

The first compile succeeds quietly; the second throws warnings out of the
gate, then crashes. And the "after" output:

  error: Cannot create static reference to method equals because object
GenMapLike$class has no source module
 error: fatal error:
      while compiling:
/scala/trunk/src/library/scala/collection/immutable/IntMap.scala
         during phase: globalPhase=mixin, enteringPhase=cleanup
      library version: version 2.11.0-20130606-052937-392858b4f6
     compiler version: version 2.11.0-20130606-055515-1856bec71b
   reconstructed args: -d /tmp

    last tree to typer: Ident(key$2)
        tree position: line 377 of
/scala/trunk/src/library/scala/collection/immutable/IntMap.scala
             tree tpe: Int
               symbol: value key$2 (flags: <param> <synthetic>)
    symbol definition: key$2: Int (a TermSymbol)
       symbol package: scala.collection.immutable
        symbol owners: value key$2 -> constructor IntMap$$anonfun$unionWith$2
-> anonymous class anonfun$unionWith$2
            call site: object IntMap$Nil in package immutable in package
immutable

  == Source file context for tree position ==

     374       }
    375     case (IntMap.Tip(key, value), x) => x.updateWith[S](key, value,
(x, y) => f(key, y, x))
    376     case (x, IntMap.Tip(key, value)) => x.updateWith[S](key, value,
(x, y) => f(key, x, y))
    377     case (IntMap.Nil, x) => x
    378     case (x, IntMap.Nil) => x
    379   }
    380