BF: annexrepo: Error when using broken 'find --json'

On git-annex 7.20190626, 'find --json' produces regular output rather
than json.  In addition to being broken, this is problematic because,
before the previous commit, we silently dropped lines that didn't
start with "{".  In the case of 'datalad get', this led to us
incorrectly claiming that an absent file was present.

We now fail with a RuntimeError in this situation, but let's raise a
more informative error when 'find --json' is used with git-annex
7.20190626.

This has been fixed in b8ef1bf3b (Fix find --json to output json once
more, 2019-07-05).

Re: http://git-annex.branchable.com/bugs/Regression_in___96__find_--json__96___output/
Re: #3510 (comment)
Closes #3513.

datalad/support/annexrepo.py

@@ -81,6 +81,7 @@
 from .exceptions import InsufficientArgumentsError
 from .exceptions import OutOfSpaceError
 from .exceptions import RemoteNotAvailableError
+from .exceptions import BrokenExternalDependency
 from .exceptions import OutdatedExternalDependency
 from .exceptions import MissingExternalDependency
 from .exceptions import IncompleteResultsError
@@ -2541,9 +2542,17 @@ def _run_annex_command_json(self, command,
                 lgr.warning("Received non-json lines for --json command: %s",
                             others)
             else:
-                raise RuntimeError(
-                    "Received no json output for --json command, only:\n{}"
-                    .format("  ".join(others)))
+                annex_ver = external_versions['cmd:annex']
+                if annex_ver == "7.20190626" and command == "find":
+                    # TODO: Drop this once GIT_ANNEX_MIN_VERSION is over
+                    # 7.20190626.
+                    exc = BrokenExternalDependency(
+                        "find --json output is broken on git-annex 7.20190626")
+                else:
+                    exc = RuntimeError(
+                        "Received no json output for --json command, only:\n{}"
+                        .format("  ".join(others)))
+                raise exc
         return json_objects
 
     # TODO: reconsider having any magic at all and maybe just return a list/dict always