fix: update error diagnostic reporting in existing Event Streams code

[kccox]

What has been done

Update the contextualized error messages in existing Event Streams code to use the flex.TerraformErrorf package, as in https://github.ibm.com/CloudEngineering/client-error-toolchain/wiki/Terraform-Support-Instructions.

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Relates OR Closes #0000

Output from acceptance testing:

% make testacc TEST=./ibm/service/eventstreams TESTARGS='-run=TestAccIBMEventStreamsTopicDataSourceBasic'
==> Checking that code complies with gofmt requirements...
TF_ACC=1 go test ./ibm/service/eventstreams -v -run=TestAccIBMEventStreamsTopicDataSourceBasic -timeout 700m
=== RUN   TestAccIBMEventStreamsTopicDataSourceBasic
--- PASS: TestAccIBMEventStreamsTopicDataSourceBasic (220.34s)
PASS
ok  	github.com/IBM-Cloud/terraform-provider-ibm/ibm/service/eventstreams	222.988s

% make testacc TEST=./ibm/service/eventstreams TESTARGS='-run=TestAccIBMEventStreamsTopicResourceWithExistingInstance'
==> Checking that code complies with gofmt requirements...
TF_ACC=1 go test ./ibm/service/eventstreams -v -run=TestAccIBMEventStreamsTopicResourceWithExistingInstance -timeout 700m
=== RUN   TestAccIBMEventStreamsTopicResourceWithExistingInstance
--- PASS: TestAccIBMEventStreamsTopicResourceWithExistingInstance (63.17s)
PASS
ok  	github.com/IBM-Cloud/terraform-provider-ibm/ibm/service/eventstreams	65.980s

% make testacc TEST=./ibm/service/eventstreams TESTARGS='-run=TestAccIBMEventStreamsTopicResourceBasic'               
==> Checking that code complies with gofmt requirements...
TF_ACC=1 go test ./ibm/service/eventstreams -v -run=TestAccIBMEventStreamsTopicResourceBasic -timeout 700m
=== RUN   TestAccIBMEventStreamsTopicResourceBasic
--- PASS: TestAccIBMEventStreamsTopicResourceBasic (77.67s)
PASS
ok  	github.com/IBM-Cloud/terraform-provider-ibm/ibm/service/eventstreams	80.384s

% make testacc TEST=./ibm/service/eventstreams TESTARGS='-run=TestAccIBMEventStreamsTopicImport'       
==> Checking that code complies with gofmt requirements...
TF_ACC=1 go test ./ibm/service/eventstreams -v -run=TestAccIBMEventStreamsTopicImport -timeout 700m
=== RUN   TestAccIBMEventStreamsTopicImport
--- PASS: TestAccIBMEventStreamsTopicImport (57.77s)
PASS
ok  	github.com/IBM-Cloud/terraform-provider-ibm/ibm/service/eventstreams	60.471s

% make testacc TEST=./ibm/service/eventstreams TESTARGS='-run=TestAccIBMEventStreamsSchemaDataSourceBasic'
==> Checking that code complies with gofmt requirements...
TF_ACC=1 go test ./ibm/service/eventstreams -v -run=TestAccIBMEventStreamsSchemaDataSourceBasic -timeout 700m
=== RUN   TestAccIBMEventStreamsSchemaDataSourceBasic
--- PASS: TestAccIBMEventStreamsSchemaDataSourceBasic (185.74s)
PASS
ok  	github.com/IBM-Cloud/terraform-provider-ibm/ibm/service/eventstreams	188.384s

% make testacc TEST=./ibm/service/eventstreams TESTARGS='-run=TestAccIBMEventStreamsSchemaBasic'          
==> Checking that code complies with gofmt requirements...
TF_ACC=1 go test ./ibm/service/eventstreams -v -run=TestAccIBMEventStreamsSchemaBasic -timeout 700m
=== RUN   TestAccIBMEventStreamsSchemaBasic
--- PASS: TestAccIBMEventStreamsSchemaBasic (74.48s)
PASS
ok  	github.com/IBM-Cloud/terraform-provider-ibm/ibm/service/eventstreams	77.159s


% make testacc TEST=./ibm/service/eventstreams TESTARGS='-run=TestAccIBMEventStreamsSchemaAllArgs'
==> Checking that code complies with gofmt requirements...
TF_ACC=1 go test ./ibm/service/eventstreams -v -run=TestAccIBMEventStreamsSchemaAllArgs -timeout 700m
=== RUN   TestAccIBMEventStreamsSchemaAllArgs
--- PASS: TestAccIBMEventStreamsSchemaAllArgs (36.94s)
PASS
ok  	github.com/IBM-Cloud/terraform-provider-ibm/ibm/service/eventstreams	39.653s


% make testacc TEST=./ibm/service/eventstreams TESTARGS='-run=TestAccIBMEventStreamsSchemaImport' 
==> Checking that code complies with gofmt requirements...
TF_ACC=1 go test ./ibm/service/eventstreams -v -run=TestAccIBMEventStreamsSchemaImport -timeout 700m
=== RUN   TestAccIBMEventStreamsSchemaImport
--- PASS: TestAccIBMEventStreamsSchemaImport (43.85s)
PASS
ok  	github.com/IBM-Cloud/terraform-provider-ibm/ibm/service/eventstreams	46.520s

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