Enable -Xlog-implicits by default for whitebox macro exceptions

[retronym]

scala> import reflect.macros._, language.experimental.macros; class C1; implicit def c1: C1 = macro c1Impl; def c1Impl(c: whitebox.Context) = {import c.universe._; throw null}
import reflect.macros._
import language.experimental.macros
defined class C1
defined term macro c1: C1
c1Impl: (c: scala.reflect.macros.whitebox.Context)Nothing

scala> implicitly[C1]
<console>:18: c1 is not a valid implicit value for C1 because:
exception typechecking implicit candidate (a whitebox macro): exception during macro expansion:
java.lang.NullPointerException
	at .c1Impl(<console>:11)

       implicitly[C1]
                 ^
<console>:18: this.<c1: error> is not a valid implicit value for C1 because:
exception typechecking implicit candidate (a whitebox macro): exception during macro expansion:
java.lang.NullPointerException
	at .c1Impl(<console>:11)

       implicitly[C1]
                 ^
<console>:18: error: could not find implicit value for parameter e: C1
       implicitly[C1]
                 ^

scala> :quit