Only generate @error message getter when defined in the model

[david-perez]

We are currently generating a useless message getter that returns
None even when the message field is not defined in the model.
Granted, in the client this field is almost always there, because the
AddErrorMessage model transformer is applied unless the user opts out
in their smithy-build.json.

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[github-actions]

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• AWS SDK (ignoring whitespace)
• Server Test (ignoring whitespace)
• Server Test Python (ignoring whitespace)

A new doc preview is ready to view.

Rust Wrk benchmark report:

Duration: 90 sec, Connections: 32, Threads: 2

Measurement	Deviation	Current	Old
Requests/sec	-6.75%	73953.96	79304.2
Total requests	-6.76%	6657432	7139775
Total errors	NaN%	0	0
Total successes	-6.76%	6657432	7139775
Average latency ms	-45.69%	0.63	1.16
Minimum latency ms	0.00%	0.02	0.02
Maximum latency ms	-28.36%	17.05	23.8
Stdev latency ms	-55.45%	0.9	2.02
Transfer Mb	-6.76%	692.04	742.18
Connect errors	NaN%	0	0
Read errors	NaN%	0	0
Write errors	NaN%	0	0
Status errors (not 2xx/3xx)	NaN%	0	0
Timeout errors	NaN%	0	0

[github-actions]

A new generated diff is ready to view.

• AWS SDK (ignoring whitespace)
• Server Test (ignoring whitespace)
• Server Test Python (ignoring whitespace)

A new doc preview is ready to view.

Rust Wrk benchmark report:

Duration: 90 sec, Connections: 32, Threads: 2

Measurement	Deviation	Current	Old
Requests/sec	7.10%	39103.79	36510.3
Total requests	7.10%	3520237	3286767
Total errors	NaN%	0	0
Total successes	7.10%	3520237	3286767
Average latency ms	-5.62%	0.84	0.89
Minimum latency ms	-33.33%	0.02	0.03
Maximum latency ms	10.32%	18.5	16.77
Stdev latency ms	3.85%	0.54	0.52
Transfer Mb	7.10%	365.93	341.66
Connect errors	NaN%	0	0
Read errors	NaN%	0	0
Write errors	NaN%	0	0
Status errors (not 2xx/3xx)	NaN%	0	0
Timeout errors	NaN%	0	0