Add CommandExt trait (#146)

* Color CI output

* Improve `CommandExt`

* Add comments explaining `#[allow]`s

* Remove useless `dead_code` annotation

* Improve error messages

* Print errors when running a shell errors

* fixup! Remove useless `dead_code` annotation

src/error.rs

@@ -4,11 +4,11 @@ use thiserror::Error;
 
 #[derive(Error, Debug, PartialEq, Eq)]
 pub enum TopgradeError {
-    #[error("{0}")]
-    ProcessFailed(ExitStatus),
+    #[error("`{0}` failed: {1}")]
+    ProcessFailed(String, ExitStatus),
 
-    #[error("{0}: {1}")]
-    ProcessFailedWithOutput(ExitStatus, String),
+    #[error("`{0}` failed: {1}")]
+    ProcessFailedWithOutput(String, ExitStatus, String),
 
     #[error("Sudo is required for this step")]
     #[allow(dead_code)]