Add more suggestions on how to deal with excluding a package from a workspace.

src/cargo/core/workspace.rs

@@ -660,7 +660,10 @@ impl<'cfg> Workspace<'cfg> {
             failure::bail!(
                 "current package believes it's in a workspace when it's not:\n\
                  current:   {}\n\
-                 workspace: {}\n\n{}",
+                 workspace: {}\n\n{}\n\
+                 Alternatively, to keep it out of the workspace, add the package \
+                 to the `workspace.exclude` array, or add an empty `[workspace]` \
+                 table to the package's manifest.",
                 self.current_manifest.display(),
                 root.display(),
                 extra

tests/testsuite/workspaces.rs

@@ -314,6 +314,7 @@ current: [..]Cargo.toml
 workspace: [..]Cargo.toml
 
 this may be fixable [..]
+[..]
 ",
         )
         .run();
@@ -779,6 +780,7 @@ workspace: [..]Cargo.toml
 
 this may be fixable by adding `bar` to the `workspace.members` array of the \
 manifest located at: [..]
+[..]
 ",
         )
         .run();
@@ -979,6 +981,7 @@ workspace: [..]
 
 this may be fixable by ensuring that this crate is depended on by the workspace \
 root: [..]
+[..]
 [CREATED] library `bar` package
 ",
         )