add sbot process stats function, struct and error variants

peach-stats/src/error.rs

@@ -1,7 +1,7 @@
 //! Custom error type for `peach-stats`.
 
 use probes::ProbeError;
-use std::{error, fmt, io::Error as IoError};
+use std::{error, fmt, io::Error as IoError, str::Utf8Error};
 
 /// Custom error type encapsulating all possible errors when retrieving system
 /// statistics.
@@ -17,6 +17,10 @@ pub enum StatsError {
     MemStat(ProbeError),
     /// Failed to retrieve system uptime.
     Uptime(IoError),
+    /// Systemctl command returned an error.
+    Systemctl(IoError),
+    /// Failed to interpret sequence of `u8` as a string.
+    Utf8String(Utf8Error),
 }
 
 impl error::Error for StatsError {}
@@ -39,6 +43,12 @@ impl fmt::Display for StatsError {
             StatsError::Uptime(ref source) => {
                 write!(f, "Failed to retrieve system uptime: {}", source)
             }
+            StatsError::Systemctl(ref source) => {
+                write!(f, "Systemctl command returned an error: {}", source)
+            }
+            StatsError::Utf8String(ref source) => {
+                write!(f, "Failed to convert stdout to string: {}", source)
+            }
         }
     }
 }