fbcode_builder: fix the error message if we cannot find a project fetcher

Summary:
Fix printing the manifest context in the error message if we cannot find a
project fetcher.  Previously the context in the message would be printed as
something like `<getdeps.manifest.ManifestContext object at 0x7fcce987e610>`,
now it shows instead as something like
`{distro=ubuntu, distro_vers=18.04, fb=off, os=linux, test=off}`

Reviewed By: chadaustin

Differential Revision: D17089208

fbshipit-source-id: c16549b61030d813b7b5ff9f65966436dc1e1898

build/fbcode_builder/getdeps/manifest.py

@@ -349,7 +349,7 @@ class ManifestParser(object):
                 )
 
         raise KeyError(
-            "project %s has no fetcher configuration matching %r" % (self.name, ctx)
+            "project %s has no fetcher configuration matching %s" % (self.name, ctx)
         )
 
     def create_builder(self, build_options, src_dir, build_dir, inst_dir, ctx):