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

facebook/fbthrift,facebook/fbthrift,facebook/fbthrift,facebook/fbthrift,facebook/fbthrift,facebook/fbthrift,facebook/fbthrift,facebook/fbthrift,facebook/fbthrift