Clearer unsupported platform error message

If we don't know how to support a detected platform the old code gave a
cryptic error message. Make this more clear by setting a specific
"unknown" platform value that raises if you attempt to get package
manager info with an error message that explains this situation.

Change-Id: I125dad3a3aa8f67ad5e96430e51022f5aa461b23

bindep/depends.py

@@ -384,6 +384,8 @@ class Depends(object):
         elif distro_id in ["arch"]:
             atoms.add("pacman")
             self.platform = Pacman()
+        else:
+            self.platform = Unknown()
         return ["platform:%s" % (atom,) for atom in sorted(atoms)]
 
 
@@ -398,6 +400,13 @@ class Platform(object):
         raise NotImplementedError(self.get_pkg_version)
 
 
+class Unknown(Platform):
+    """Unknown platform implementation. Raises error."""
+
+    def get_pkg_version(self, pkg_name):
+        raise Exception("Uknown package manager for current platform.")
+
+
 class Brew(Platform):
     """brew specific platform implementation."""
 

bindep/tests/fixtures/unknown/etc/os-release

@@ -0,0 +1,10 @@
+NAME="Unknown"
+VERSION="12345"
+ID=unknown
+ID_LIKE=unknown
+PRETTY_NAME="Not a real distro 12345"
+VERSION_ID="12345"
+HOME_URL="https://example.com"
+SUPPORT_URL="https://help.example.com"
+BUG_REPORT_URL="https://bugs.example.com"
+VERSION_CODENAME=Testing

bindep/tests/test_depends.py

@@ -25,6 +25,7 @@ import distro
 import fixtures
 import mock
 import ometa.runtime
+from testtools import ExpectedException
 from testtools.matchers import Contains
 from testtools.matchers import Equals
 from testtools.matchers import MatchesSetwise
@@ -93,6 +94,16 @@ class TestDepends(TestCase):
         yield mock_checkoutput
         mock_checkoutput.assert_called_once_with()
 
+    def test_detects_unknown(self):
+        with DistroFixture("Unknown"):
+            depends = Depends("")
+            self.assertThat(
+                depends.platform_profiles(), Contains("platform:unknown"))
+            with ExpectedException(Exception,
+                                   "Uknown package manager for "
+                                   "current platform."):
+                depends.platform.get_pkg_version('x')
+
     def test_detects_amazon_linux(self):
         with DistroFixture("AmazonAMI"):
             depends = Depends("")