errors: Use common.ErrMsg for constants

[lukedirtwalker]

This helps to move to a more friendly go 1.13 error environment:

1. the error constants can be detected with xerrors.Is.
2. since common.ErrMsg is also an error we can easily migrate common.NewBasicError calls towards the corresponding serrors calls.
3. in the future we can replace all common.ErrMsg in one go with serrors.New.

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This change is 

[oncilla]

Reviewed 89 of 89 files at r1.
Reviewable status: all files reviewed, 10 unresolved discussions (waiting on @lukedirtwalker)

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go/cert_srv/setup.go, line 43 at r1 (raw file):

const (
	ErrorConf      common.ErrMsg = "Unable to load configuration"

Err

---

go/cert_srv/internal/config/config.go, line 43 at r1 (raw file):

	ReissueReqTimeout = 5 * time.Second

	ErrorKeyConf   common.ErrMsg = "Unable to load KeyConf"

Err

---

go/lib/keyconf/keyconf.go, line 72 at r1 (raw file):

// Errors
const (
	ErrorOpen    common.ErrMsg = "Unable to load key"

ErrOpen same below

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go/lib/scrypto/validity.go, line 33 at r1 (raw file):

	ErrNotBeforeNotSet = errors.New("not_before not set")
	// ErrErrInvalidValidityPeriod indicates an invalid validity period.
	ErrErrInvalidValidityPeriod = errors.New("not_after before not_before")

ErrErr?

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go/lib/scrypto/version.go, line 28 at r1 (raw file):

// ErrErrInvalidVersion indicates an invalid trust file version.
var ErrErrInvalidVersion = errors.New("version must not be zero")

ErrErr?

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go/lib/scrypto/cert/chain.go, line 242 at r1 (raw file):

	}
	if err = validateFields(m, chainFields); err != nil {
		return common.NewBasicError(ErrUnableValidateFields, err)

ErrValidatingFields

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go/lib/scrypto/trc/entries.go, line 49 at r1 (raw file):

	}
	if err = validateFields(m, coreASFields); err != nil {
		return common.NewBasicError(ErrUnableValidateFields, err)

ErrValidatingFields

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go/lib/scrypto/trc/trc.go, line 42 at r1 (raw file):

// Error strings
const (
	EarlyUsage          common.ErrMsg = "Creation time in the future"

Err all the things

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go/lib/spkt/cmnhdr.go, line 42 at r1 (raw file):

const (
	ErrorUnsuppVersion common.ErrMsg = "Unsupported SCION version"

Err

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go/lib/topology/raw.go, line 32 at r1 (raw file):

const (
	ErrorOpen    common.ErrMsg = "Unable to open topology"

Err

[lukedirtwalker]

Reviewable status: 74 of 88 files reviewed, 10 unresolved discussions (waiting on @oncilla)

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go/cert_srv/setup.go, line 43 at r1 (raw file):

Previously, Oncilla wrote…

Err

Done.

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go/cert_srv/internal/config/config.go, line 43 at r1 (raw file):

Previously, Oncilla wrote…

Err

Done.

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go/lib/keyconf/keyconf.go, line 72 at r1 (raw file):

Previously, Oncilla wrote…

ErrOpen same below

Done.

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go/lib/scrypto/validity.go, line 33 at r1 (raw file):

Previously, Oncilla wrote…

ErrErr?

Done.

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go/lib/scrypto/version.go, line 28 at r1 (raw file):

Previously, Oncilla wrote…

ErrErr?

Done.

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go/lib/scrypto/cert/chain.go, line 242 at r1 (raw file):

Previously, Oncilla wrote…

ErrValidatingFields

Done.

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go/lib/scrypto/trc/entries.go, line 49 at r1 (raw file):

Previously, Oncilla wrote…

ErrValidatingFields

Done.

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go/lib/scrypto/trc/trc.go, line 42 at r1 (raw file):

Previously, Oncilla wrote…

Err all the things

Done.

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go/lib/spkt/cmnhdr.go, line 42 at r1 (raw file):

Previously, Oncilla wrote…

Err

Done.

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go/lib/topology/raw.go, line 32 at r1 (raw file):

Previously, Oncilla wrote…

Err

Done.

[oncilla]

Reviewed 16 of 16 files at r2.
Reviewable status: complete! all files reviewed, all discussions resolved

[oncilla]

Reviewable status: complete! all files reviewed, all discussions resolved