Fixes #4 Improve error reporting

__tests__/GithubError.test.ts

@@ -0,0 +1,88 @@
+import { GithubError } from "../src/GithubError"
+
+describe('GithubError', () => {
+    it('generates missing resource error message', () => {
+        const resource = "release"
+        const error = {
+            code: "missing",
+            resource: resource
+        }
+
+        const githubError = new GithubError(error)
+        const message = githubError.toString()
+
+        expect(message).toBe(`${resource} does not exist.`)
+    })
+
+    it('generates missing field error message', () => {
+        const resource = "release"
+        const field = "body"
+        const error = {
+            code: "missing_field",
+            field: field,
+            resource: resource
+        }
+
+        const githubError = new GithubError(error)
+        const message = githubError.toString()
+
+        expect(message).toBe(`The ${field} field on ${resource} is missing.`)
+    })
+
+    it('generates invalid field error message', () => {
+        const resource = "release"
+        const field = "body"
+        const error = {
+            code: "invalid",
+            field: field,
+            resource: resource
+        }
+
+        const githubError = new GithubError(error)
+        const message = githubError.toString()
+
+        expect(message).toBe(`The ${field} field on ${resource} is an invalid format.`)
+    })
+
+    it('generates resource already exists error message', () => {
+        const resource = "release"
+        const field = "body"
+        const error = {
+            code: "already_exists",
+            resource: resource
+        }
+
+        const githubError = new GithubError(error)
+        const message = githubError.toString()
+
+        expect(message).toBe(`${resource} already exists.`)
+    })
+
+    describe('generates custom error message', () => {
+        it('with documentation url', () => {
+            const url = "https://api.example.com"
+            const error = {
+                code: "custom",
+                message: "foo",
+                documentation_url: url
+            }
+    
+            const githubError = new GithubError(error)
+            const message = githubError.toString()
+    
+            expect(message).toBe(`foo\nPlease see ${url}.`)
+        })
+
+        it('without documentation url', () => {
+            const error = {
+                code: "custom",
+                message: "foo"
+            }
+    
+            const githubError = new GithubError(error)
+            const message = githubError.toString()
+    
+            expect(message).toBe('foo')
+        })
+    })
+})