Tweak the error message when updating an empty registry while offline.

Author: ehuss

src/cargo/sources/registry/remote.rs

@@ -177,10 +177,14 @@ impl<'cfg> RegistryData for RemoteRegistry<'cfg> {
     fn update_index(&mut self) -> CargoResult<()> {
         if self.config.cli_unstable().offline {
             if self.repo()?.is_empty()? {
+                // An empty repository is guaranteed to fail, since hitting
+                // this path means we need at least one crate. This is an
+                // attempt to provide a better error message other than "no
+                // matching package named …".
                 failure::bail!(
                     "unable to fetch {} in offline mode\n\
-                     Run `cargo fetch` to download the registry index and all \
-                     of a project's dependencies before going offline.",
+                     Try running without the offline flag, or try running \
+                     `cargo fetch` within your project directory before going offline.",
                     self.source_id
                 );
             }

tests/testsuite/offline.rs

@@ -178,7 +178,8 @@ Caused by:
 
 Caused by:
   unable to fetch registry `https://github.com/rust-lang/crates.io-index` in offline mode
-Run `cargo fetch` to download the registry index and all of a project's dependencies before going offline.
+Try running without the offline flag, or try running `cargo fetch` within your \
+project directory before going offline.
 ",
         )
         .run();