SI-7418 Avoid concurrent use of compiler in REPL startup

Enable the tab completer *after* we're finished binding
$intp and unleashing power mode on the asynchronous
REPL startup thread.

Tested manually:
 - run qbin/scala
 - Paste "".toUp
 - Hit <TAB> like a maniac

Before this patch, the crash was reproducible almost every time.
Afterwards, not the once.
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
Merge pull request #2638 from paulp/pr/error-output

Cleanup of crash output.
Merge pull request #2627 from retronym/ticket/7418

SI-7418 Avoid concurrent use of compiler in REPL startup