Add extra error logging to seek_wait() method

lib/tarantool_server.py

@@ -464,14 +464,20 @@ def seek_wait(self, msg, proc=None, name=None, deadline=None):
                 break
             gevent.sleep(0.001)
         else:
+            color_stdout(
+                "Logfile {} does not exist".format(self.path), schema='error'
+            )
             return False
 
         with open(self.path, 'r') as f:
             f.seek(self.log_begin, os.SEEK_SET)
             cur_pos = self.log_begin
             while not deadline or time.time() < deadline:
                 if not (proc is None):
+                    # proc.poll() returns None if the process is working. When
+                    # the process completed, it returns the process exit code.
                     if not (proc.poll() is None):
+                        # Raise an error since the process completed.
                         raise TarantoolStartError(name)
                 log_str = f.readline()
                 if not log_str:
@@ -481,6 +487,12 @@ def seek_wait(self, msg, proc=None, name=None, deadline=None):
                 if re.findall(msg, log_str):
                     return True
                 cur_pos = f.tell()
+
+        color_stdout(
+            "Failed to find '{}' pattern in {} logfile".format(msg, self.path),
+            schema='error',
+        )
+
         return False