Wrap error returns in proccesses collector

Wrap error returns so that the user can know more about what failed.
Returns from getAllocatedThreads() are errors anyway.

Related to #2110

Signed-off-by: Ben Kochie <superq@gmail.com>

collector/processes_linux.go

@@ -105,7 +105,7 @@ func (c *processCollector) Update(ch chan<- prometheus.Metric) error {
 func (c *processCollector) getAllocatedThreads() (int, map[string]int32, int, error) {
 	p, err := c.fs.AllProcs()
 	if err != nil {
-		return 0, nil, 0, err
+		return 0, nil, 0, fmt.Errorf("unable to list all processes: %w", err)
 	}
 	pids := 0
 	thread := 0
@@ -119,7 +119,7 @@ func (c *processCollector) getAllocatedThreads() (int, map[string]int32, int, er
 				continue
 			}
 			level.Debug(c.logger).Log("msg", "error reading stat for pid", "pid", pid, "err", err)
-			return 0, nil, 0, err
+			return 0, nil, 0, fmt.Errorf("error reading stat for pid %d: %w", pid, err)
 		}
 		pids++
 		procStates[stat.State]++