More helpful errors when we can't get a distribution in offline mode.

src/zc/buildout/easy_install.py

@@ -422,7 +422,12 @@ class Installer:
         dist, avail = self._satisfied(requirement)
 
         if dist is None:
-            if self._dest is not None:
+            if self._dest is None:
+                raise zc.buildout.UserError(
+                    "We don't have a distribution for %s\n"
+                    "and can't install one in offline (no-install) mode.\n"
+                    % requirement)
+
             logger.info(*__doing__)
 
             # Retrieve the dist:
@@ -616,6 +621,13 @@ class Installer:
                 "Couldn't find a source distribution for %r."
                 % str(requirement))
 
+        if self._dest is None:
+            raise zc.buildout.UserError(
+                "We don't have a distribution for %s\n"
+                "and can't build one in offline (no-install) mode.\n"
+                % requirement
+                )
+
         logger.debug('Building %r', spec)
 
         tmp = self._download_cache

src/zc/buildout/tests.py

@@ -2806,6 +2806,31 @@ def cant_use_install_from_cache_and_offline_together():
     <BLANKLINE>
     """
 
+def error_installing_in_offline_mode_if_dont_have_needed_dist():
+    r"""
+    >>> import zc.buildout.easy_install
+    >>> ws = zc.buildout.easy_install.install(
+    ...     ['demo==0.2'], None,
+    ...     links=[link_server], index=link_server+'index/')
+    Traceback (most recent call last):
+    ...
+    UserError: We don't have a distribution for demo==0.2
+    and can't install one in offline (no-install) mode.
+    <BLANKLINE>
+    """
+
+def error_building_in_offline_mode_if_dont_have_needed_dist():
+    r"""
+    >>> zc.buildout.easy_install.build(
+    ...   'extdemo', None,
+    ...   {}, links=[link_server], index=link_server+'index/')
+    Traceback (most recent call last):
+    ...
+    UserError: We don't have a distribution for extdemo
+    and can't build one in offline (no-install) mode.
+    <BLANKLINE>
+    """
+
 
 ######################################################################