resource/github_*: Prevent crashing on invalid ID format

[radeksimko]

Fixes #11
Fixes #49

Acceptance tests

make testacc TEST=./github
==> Checking that code complies with gofmt requirements...
TF_ACC=1 go test ./github -v  -timeout 120m
=== RUN   TestAccGithubIpRangesDataSource_existing
--- PASS: TestAccGithubIpRangesDataSource_existing (1.07s)
=== RUN   TestAccGithubTeamDataSource_noMatchReturnsError
--- PASS: TestAccGithubTeamDataSource_noMatchReturnsError (0.14s)
=== RUN   TestAccGithubUserDataSource_noMatchReturnsError
--- PASS: TestAccGithubUserDataSource_noMatchReturnsError (0.14s)
=== RUN   TestAccGithubUserDataSource_existing
--- PASS: TestAccGithubUserDataSource_existing (2.31s)
=== RUN   TestProvider
--- PASS: TestProvider (0.00s)
=== RUN   TestProvider_impl
--- PASS: TestProvider_impl (0.00s)
=== RUN   TestAccGithubBranchProtection_basic
--- PASS: TestAccGithubBranchProtection_basic (8.04s)
=== RUN   TestAccGithubBranchProtection_emptyItems
--- PASS: TestAccGithubBranchProtection_emptyItems (3.95s)
=== RUN   TestAccGithubBranchProtection_importBasic
--- PASS: TestAccGithubBranchProtection_importBasic (3.99s)
=== RUN   TestAccGithubIssueLabel_basic
--- PASS: TestAccGithubIssueLabel_basic (4.84s)
=== RUN   TestAccGithubIssueLabel_existingLabel
--- PASS: TestAccGithubIssueLabel_existingLabel (3.29s)
=== RUN   TestAccGithubIssueLabel_importBasic
--- PASS: TestAccGithubIssueLabel_importBasic (3.99s)
=== RUN   TestAccGithubMembership_basic
--- PASS: TestAccGithubMembership_basic (2.02s)
=== RUN   TestAccGithubMembership_importBasic
--- PASS: TestAccGithubMembership_importBasic (1.47s)
=== RUN   TestAccGithubOrganizationWebhook_basic
--- PASS: TestAccGithubOrganizationWebhook_basic (2.42s)
=== RUN   TestAccGithubRepositoryCollaborator_basic
--- PASS: TestAccGithubRepositoryCollaborator_basic (4.26s)
=== RUN   TestAccGithubRepositoryCollaborator_importBasic
--- PASS: TestAccGithubRepositoryCollaborator_importBasic (4.23s)
=== RUN   TestAccGithubRepositoryDeployKey_basic
--- PASS: TestAccGithubRepositoryDeployKey_basic (3.33s)
=== RUN   TestAccGithubRepositoryDeployKey_importBasic
--- PASS: TestAccGithubRepositoryDeployKey_importBasic (4.12s)
=== RUN   TestAccGithubRepository_basic
--- FAIL: TestAccGithubRepository_basic (6.29s)
	testing.go:518: Step 1 error: Check failed: Check 2/2 error: got description "Terraform acceptance tests 0qk9jewsxy"; want "Updated Terraform acceptance tests 0qk9jewsxy"
=== RUN   TestAccGithubRepository_archive
--- PASS: TestAccGithubRepository_archive (2.80s)
=== RUN   TestAccGithubRepository_archiveUpdate
--- PASS: TestAccGithubRepository_archiveUpdate (3.92s)
=== RUN   TestAccGithubRepository_importBasic
--- PASS: TestAccGithubRepository_importBasic (2.69s)
=== RUN   TestAccGithubRepository_defaultBranch
--- PASS: TestAccGithubRepository_defaultBranch (5.39s)
=== RUN   TestAccGithubRepository_templates
--- PASS: TestAccGithubRepository_templates (3.93s)
=== RUN   TestAccGithubRepository_topics
--- PASS: TestAccGithubRepository_topics (6.23s)
=== RUN   TestAccGithubRepositoryWebhook_basic
--- PASS: TestAccGithubRepositoryWebhook_basic (8.26s)
=== RUN   TestAccGithubRepositoryWebhook_importBasic
--- FAIL: TestAccGithubRepositoryWebhook_importBasic (2.10s)
	testing.go:518: Step 0 error: Error applying: 1 error(s) occurred:

		* github_repository_webhook.foo: 1 error(s) occurred:

		* github_repository_webhook.foo: POST https://api.github.com/repos/terraformtesting/foo-qranak8c8e/hooks: 404 Not Found []
=== RUN   TestAccGithubTeamMembership_basic
--- PASS: TestAccGithubTeamMembership_basic (5.36s)
=== RUN   TestAccGithubTeamMembership_importBasic
--- PASS: TestAccGithubTeamMembership_importBasic (2.52s)
=== RUN   TestAccGithubTeamRepository_basic
--- PASS: TestAccGithubTeamRepository_basic (7.06s)
=== RUN   TestAccGithubTeamRepository_importBasic
--- PASS: TestAccGithubTeamRepository_importBasic (4.34s)
=== RUN   TestAccCheckGetPermissions
--- PASS: TestAccCheckGetPermissions (0.00s)
=== RUN   TestAccGithubTeam_basic
--- PASS: TestAccGithubTeam_basic (4.58s)
=== RUN   TestAccGithubTeam_hierarchical
--- PASS: TestAccGithubTeam_hierarchical (3.70s)
=== RUN   TestAccGithubTeam_importBasic
--- PASS: TestAccGithubTeam_importBasic (1.49s)
=== RUN   TestAccGithubUtilRole_validation
--- PASS: TestAccGithubUtilRole_validation (0.00s)
=== RUN   TestAccGithubUtilTwoPartID
--- PASS: TestAccGithubUtilTwoPartID (0.00s)
FAIL
FAIL	github.com/terraform-providers/terraform-provider-github/github	124.381s
make: *** [testacc] Error 1

Failures above are related to eventually consistent API.

[bflad]

LGTM