Try harder to avoid reporting unpositioned errors

Rather than issuing an error at NoPosition, which usually means
an unpositioned tree is being typechecked, walk up the context
chain for an enclosing positioned tree.

I've tested this manually with ensime which was getting an
unpositioned warning as a result of a unpositioned trees created
by a macro in scalatest.

```
% ../scala-positioned-error/build/quick/bin/scalac  @args.txt
/Users/jason/code/ensime-server/core/src/it/scala/org/ensime/core/RichPresentationCompilerSpec.scala:216: warning: implicit numeric widening
    ) { (p, cc) =>
                ^
```

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