[BEAM-7154] Updating Go SDK errors (Part 3 - Final)

[youngoli]

Updating standard Go error functionality (errors.New and fmt.Errorf) to use the newly added Beam "errors" package. This PR covers the core subdirectory under sdks/go/pkg/beam.

I tried to keep the actual changes as straightforward as possible:

1. If a call isn't wrapping another error, replace it with New or Errorf. This should be functionally identical.
2. If a call is wrapping another error, replace it with either Wrap or WithContext using my best judgement. I generally used Wrap when the wrapped error originates from third-party code (i.e. any non-Beam package), and I tried to use WithContext otherwise.
3. WithContext messages make the most sense in present tense (e.g. "doing foo", "reading bar"). This means I had to adjust some messages slightly, as little as I could without losing any useful information.
4. For existing error messages that combined context and errors, I moved the context part to a separate line using WithContext, to improve legibility.

This is the last PR for migrating to the new errors package.

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Post-Commit Tests Status (on master branch)

Lang	SDK	Apex	Dataflow	Flink	Gearpump	Samza	Spark
Go		---	---	---	---	---	---
Java
Python		---			---	---	---

Pre-Commit Tests Status (on master branch)

---	Java	Python	Go	Website
Non-portable
Portable	---		---	---

See .test-infra/jenkins/README for trigger phrase, status and link of all Jenkins jobs.

[youngoli]

R: @lostluck

[lostluck]

LGTM
R: @aaltay please merge. Thanks!