This is a backport of d8ba3fe.

As `DataCollector` always returns the data it managed to collect
successfully and passes errors as a separate object, `connectAndInspect`
has put errors as `_errors` property of the result to be consistent.
This was a horrible design decision that was counter-intuitive and would
only lead to the lack of proper error handling in application code (and
has actually led in our integration test).

Worse than that, even this `_errors` property would never have been
populated with errors because of a bug in `connectAndInspect`.  A patch
for the bug (just in case we ever want to backport it):

diff --git a/lib/client.js b/lib/client.js
index 83236d9..b0436fd 100644
--- a/lib/client.js
+++ b/lib/client.js
@@ -124,7 +124,7 @@ Client.prototype.connectAndInspect = function(
       interfaces.forEach((interfaceName) => {
         connection.inspectInterface(interfaceName, (error, appInterface) => {
           if (error) {
-            appInterface = null;
+            appInterface = error;
           }
           collector.collect(interfaceName, appInterface);
         });

The proper solution would have been to stop inspecting interfaces as
soon as the first error occurs and return it as the first argument of
the callback.  As it turned out, it was quite tricky to do with
DataCollector, and some other issues were found while inspecting
`metasync` sources too.  Fixing these problems requires
backwards-incompatible changes and will certainly happen in the next
semver-major release of `metasync`, but we need a quick fix here.  For
this reason, the function was refactored to use promises.

PR-URL: #106
Reviewed-by: Mykola Bilochub <nbelochub@gmail.com>

PR-URL: #103
Reviewed-by: Mykola Bilochub <nbelochub@gmail.com>

PR-URL: #119
Reviewed-By: Mykola Bilochub <nbelochub@gmail.com>