extensions: raise when trying to find an extension that failed to load

extensions that depend on other extensions (such as record) use this pattern
to check if the dependant extension is available:

    try:
        mq = extensions.find('mq')
    except KeyError:
        return

but since if an error occurs while loading an extension it leaves its entry
in the _extensions map as None, we want to raise in that situation too.

(rather than adding another check if the return value is None)

mercurial/extensions.py

@@ -21,13 +21,17 b' def extensions():'
 
 def find(name):
     '''return module with given extension name'''
+    mod = None
     try:
-        return _extensions[name]
+        mod =  _extensions[name]
     except KeyError:
         for k, v in _extensions.iteritems():
             if k.endswith('.' + name) or k.endswith('/' + name):
-                return v
+                mod = v
+                break
+    if not mod:
         raise KeyError(name)
+    return mod
 
 def loadpath(path, module_name):
     module_name = module_name.replace('.', '_')

tests/test-qrecord.t

@@ -99,9 +99,24 b' qrecord patch (mq not present)'
   abort: 'mq' extension not loaded
   [255]
 
+help (bad mq)
+
+  $ echo "mq=nonexistant" >> $HGRCPATH
+  $ hg help qrecord
+  *** failed to import extension mq from nonexistant: [Errno 2] No such file or directory
+  hg qrecord [OPTION]... PATCH [FILE]...
+  
+  interactively record a new patch
+  
+      See "hg help qnew" & "hg help record" for more information and usage.
+  
+  use "hg -v help qrecord" to show global options
+
 help (mq present)
 
-  $ echo "mq="              >> $HGRCPATH
+  $ sed 's/mq=nonexistant/mq=/' $HGRCPATH > hgrc.tmp
+  $ mv hgrc.tmp $HGRCPATH
+
   $ hg help qrecord
   hg qrecord [OPTION]... PATCH [FILE]...