New PreviewManager to handle generating previews asynchronously (#86)

* Add PreviewManager

* Refactoring and better error handling

* Integrate PreviewManager into api_server.py

* Integrate PreviewManager into distributed_job_manager.py

* Add method to preview_manager.py to delete previews and integrate it into api_server

* Misc logging improvements

* Misc code cleanup

* Replace existing preview on job completion - Minor code fixes

src/engines/core/base_worker.py

@@ -99,6 +99,9 @@ class BaseRenderWorker(Base):
         self.__process = None
         self.last_output = None
 
+    def __repr__(self):
+        return f"<{self.__class__.__name__}|{self.id}|{self.name}|{self.status}|{self.input_path}>"
+
     @property
     def total_frames(self):
         return (self.end_frame or self.project_length) - self.start_frame + 1
@@ -131,9 +134,8 @@ class BaseRenderWorker(Base):
         generated_args = [str(x) for x in self.generate_worker_subprocess()]
         generated_args_flags = [x for x in generated_args if x.startswith('-')]
         if len(generated_args_flags) != len(set(generated_args_flags)):
-            msg = "Cannot generate subprocess - Multiple arg conflicts detected"
+            msg = f"Cannot generate subprocess - Multiple arg conflicts detected: {generated_args}"
             logger.error(msg)
-            logger.debug(f"Generated args for subprocess: {generated_args}")
             raise ValueError(msg)
         return generated_args