add addendum to implicit search failure for failed Mirror synthesis

[bishabosha]

I think it would be desirable to indicate why implicit search for a mirror fails.

tasks:

• in Synthesizer collect reasons for why synthesis fails (indicated by when a branch returns EmptyTree) - (whyNotGenericSum/whyNotGenericProduct) are good starts because they already generate error message strings that are later ignored.
• add the collected errors to the implicit search failure message that is emitted in Implicits

ideas for implementation:

• synthesizer.tryAll returns a tuple of (Tree, List[String]), where non-empty list indicates errors, then any errors are attached to the SearchFailureType of the tree returned from inferImplicitArg

[szymon-rd]

I'm taking on this issue.