Cleanup state reporting

[anbraten]

Extracted from #3849 to just have all the renamings and logging cleanups for the beginning

Changes

• [x] use separate WorkflowState and StepState types (quite some attributes were assigned but never used)
• [x] rename vars to improve readability (exp id => workflowID)
• [x] as WorkflowStates never handled exit-codes apart from logging them, the error handling in the runnger.go has slightly changed

[codecov[bot]]

Codecov Report

Attention: Patch coverage is 3.90625% with 246 lines in your changes missing coverage. Please review.

Project coverage is 27.93%. Comparing base (b8b6efb) to head (794c5b5). Report is 7 commits behind head on main.

:exclamation: Current head 794c5b5 differs from pull request most recent head 4714ac2

Please upload reports for the commit 4714ac2 to get more accurate results.

Files	Patch %	Lines
pipeline/rpc/proto/woodpecker.pb.go	0.00%	134 Missing :warning:
agent/runner.go	0.00%	40 Missing :warning:
server/grpc/rpc.go	0.00%	34 Missing :warning:
agent/rpc/client_grpc.go	0.00%	17 Missing :warning:
server/grpc/server.go	0.00%	11 Missing :warning:
agent/tracer.go	0.00%	3 Missing :warning:
server/pipeline/workflow_status.go	0.00%	3 Missing :warning:
pipeline/tracer.go	0.00%	2 Missing :warning:
agent/logger.go	0.00%	1 Missing :warning:
pipeline/pipeline.go	0.00%	1 Missing :warning:

Additional details and impacted files

```diff @@ Coverage Diff @@ ## main #3850 +/- ## ========================================== - Coverage 28.25% 27.93% -0.32% ========================================== Files 363 366 +3 Lines 25114 25590 +476 ========================================== + Hits 7095 7148 +53 - Misses 17486 17787 +301 - Partials 533 655 +122 ```

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