Use error message wording from @jimfulton

src/zc/buildout/allow-unknown-extras.txt

@@ -40,7 +40,7 @@ Now we can run the buildout and see that it fails:
     ...
     While:
       Installing eggs.
-    Error: Couldn't find the required extras. Add 'allow-unknown-extras=true' to the [buildout] configuration if this is acceptable.
+    Error: Couldn't find the required extra...
 
 If we flip the option on, the buildout succeeds
 
@@ -57,7 +57,7 @@ If we flip the option on, the buildout succeeds
     ... ''')
 
 
-Now we can run the buildout and make sure all attempts to dist.plone.org fails::
+Now we can run the buildout and only get a warning::
 
     >>> print_(system(buildout), end='') # doctest: +ELLIPSIS
     Develop: '/sample-buildout/allowdemo'

src/zc/buildout/easy_install.py

@@ -749,8 +749,13 @@ class Installer(object):
             if missing_requested:
                 if not self._allow_unknown_extras:
                     raise zc.buildout.UserError(
-                        "Couldn't find the required extras. Add 'allow-unknown-extras=true' "
-                        "to the [buildout] configuration if this is acceptable."
+                        "Couldn't find the required extra. "
+                        "This means the requirement is incorrect. "
+                        "If the requirement is itself from software you "
+                        "requested, then there might be a bug in "
+                        "requested software. You can ignore this by "
+                        "using 'allow-unknown-extras=true', however "
+                        "that may simply cause needed software to be omitted."
                     )
 
                 extra_requirements = sorted(

src/zc/buildout/easy_install.txt

@@ -225,7 +225,7 @@ is an error.
     ...     index=link_server+'index/')
     Traceback (most recent call last):
     ...
-    UserError: Couldn't find the required extras. Add 'allow-unknown-extras=true' to the [buildout] configuration if this is acceptable.
+    UserError: Couldn't find the required extra...
 
 We can pass the ``allow_unknown_extras`` argument to force the
 installation to proceed.