[BEAM-7087] Creating "errors" package internal to Beam Go SDK

[youngoli]

I'm creating a package that prints well formatted errors for Beam. In
particular this package differentiates between context and error
messages and allows the top-level error message to be sent any time
in the chain of errors and still be the first error to display.

Note that this only adds the package, I will be converting existing
errors into this package in the future.

Example of an old error:

creating new DoFn in scope root/CountWords: binding fn main.extractWordsFn: binding params [{Value int}] to input string: string is not assignable to int

New error:

string is not assignable to int
Caused by:
    creating new DoFn in scope root/CountWords:
    binding fn main.extractWordsFn:
    binding params [{Value int}] to input string:
string is not assignable to int

---

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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
Looking forward for this to be in action!
R: @aaltay Please review and merge.