_plugincontext.py: Fix unhandled exception if plugin is not foundsam/plugin-error

This fixes an AttributeError triggered if we try to import a plugin from
a package which is installed but does not contain that plugin.

Before:

    [--:--:--] START   Resolving pipeline
    Traceback (most recent call last):
      ...
    AttributeError: 'NoneType' object has no attribute 'dist'

After:

    [--:--:--] START   Resolving pipeline
    [00:00:00] FAILURE Resolving pipeline
    Error loading pipeline: Pip package buildstream-external does not contain a plugin named 'nonexistant-plugin'

1 files changed, 5 insertions, 0 deletions

diff --git a/buildstream/_plugincontext.py b/buildstream/_plugincontext.py
index 0e1231e63..264adcb79 100644
--- a/buildstream/_plugincontext.py
+++ b/buildstream/_plugincontext.py
@@ -92,6 +92,11 @@ class PluginContext():
             except pkg_resources.DistributionNotFound as e:
                 raise PluginError("Failed to load {} plugin '{}': {}"
                                   .format(self.base_type.__name__, kind, e)) from e
+
+            if package is None:
+                raise PluginError("Pip package {} does not contain a plugin named '{}'"
+                                  .format(package_name, kind))
+
             location = package.dist.get_resource_filename(
                 pkg_resources._manager,
                 package.module_name.replace('.', os.sep) + '.py'