Added two typo fixes from #137.

The pull request itself contains a change that should not be applied.

src/zc/buildout/easy_install.py

@@ -503,7 +503,7 @@ class Installer:
 
                 if dist is None:
                     raise zc.buildout.UserError(
-                        "Couln't download distribution %s." % avail)
+                        "Couldn't download distribution %s." % avail)
 
                 if dist.precedence == pkg_resources.EGG_DIST:
                     # It's already an egg, just fetch it into the dest

src/zc/buildout/testing.py

@@ -453,7 +453,7 @@ def wait(port, up):
         if up:
             raise
         else:
-            raise SystemError("Couln't stop server")
+            raise SystemError("Couldn't stop server")
 
 def install(project, destination):
     if not isinstance(destination, str):