Make error messages nicer (#551)

* Remove unhelpful information from errors

Before:

```
Git repositories failed:
   0: error: cannot pull with rebase: You have unstaged changes.
      error: Please commit or stash them.
   0:

Location:
   src/steps/git.rs:39

Backtrace omitted. Run with RUST_BACKTRACE=1 environment variable to display it.
Run with RUST_BACKTRACE=full to include source snippets.
```

After:

```
Git repositories failed:
   0: Failed to pull /Users/wiggles/.dotfiles
   1: error: cannot pull with rebase: You have unstaged changes.
      error: Please commit or stash them.

Location:
   src/steps/git.rs:39
```

* Improve git_repos errors

This removes the extra blank "0:" line at the end of the error, doesn't
print the error message twice, and provides the repo path in the error
message.

src/main.rs

@@ -47,7 +47,7 @@ pub static XDG_DIRS: Lazy<Xdg> = Lazy::new(|| Xdg::new().expect("No home directo
 pub static WINDOWS_DIRS: Lazy<Windows> = Lazy::new(|| Windows::new().expect("No home directory"));
 
 fn run() -> Result<()> {
-    color_eyre::install()?;
+    install_color_eyre()?;
     ctrlc::set_handler();
 
     let opt = CommandLineArgs::parse();
@@ -571,3 +571,16 @@ fn install_tracing(filter_directives: &str) -> Result<()> {
 
     Ok(())
 }
+
+fn install_color_eyre() -> Result<()> {
+    color_eyre::config::HookBuilder::new()
+        // Don't display the backtrace reminder by default:
+        //   Backtrace omitted. Run with RUST_BACKTRACE=1 environment variable to display it.
+        //   Run with RUST_BACKTRACE=full to include source snippets.
+        .display_env_section(false)
+        // Display location information by default:
+        //   Location:
+        //      src/steps.rs:92
+        .display_location_section(true)
+        .install()
+}