add no_home_dir error

src/error.rs

@@ -20,6 +20,7 @@ pub enum SbotCliError {
     InvalidUtf8(Utf8Error),
     // external errors
     InvalidRegex(RegexError),
+    NoHomeDir,
 }
 
 impl error::Error for SbotCliError {}
@@ -57,6 +58,12 @@ impl fmt::Display for SbotCliError {
             SbotCliError::InvalidRegex(ref err) => {
                 write!(f, "{}", err)
             }
+            SbotCliError::NoHomeDir => {
+                write!(
+                    f,
+                    "Error initialising sbotcli: no home directory could be determined"
+                )
+            }
         }
     }
 }