Add error package and some Matrix errors (#13)

Terse function names are used to make usage not stutter. For example:

```go
err := error.Forbidden("you shall not pass")
```

At the moment they are all the same fundamental `MatrixError` type. This will
be bad if we ever want to `switch` based on the kind of error. I'm hoping we
won't ever need to introspect into errors like this: ideally these errors would
be created purely for immediately being returned in an HTTP response.

`MatrixError` implements the `error` interface.

src/github.com/matrix-org/dendrite/clientapi/jsonerror/jsonerror.go

+package jsonerror
+
+import "fmt"
+
+// MatrixError represents the "standard error response" in Matrix.
+// http://matrix.org/docs/spec/client_server/r0.2.0.html#api-standards
+type MatrixError struct {
+	ErrCode string `json:"errcode"`
+	Err     string `json:"error"`
+}
+
+func (e *MatrixError) Error() string {
+	return fmt.Sprintf("%s: %s", e.ErrCode, e.Err)
+}
+
+// Forbidden is an error when the client tries to access a resource
+// they are not allowed to access.
+func Forbidden(msg string) *MatrixError {
+	return &MatrixError{"M_FORBIDDEN", msg}
+}
+
+// BadJSON is an error when the client supplies malformed JSON.
+func BadJSON(msg string) *MatrixError {
+	return &MatrixError{"M_BAD_JSON", msg}
+}
+
+// NotJSON is an error when the client supplies something that is not JSON
+// to a JSON endpoint.
+func NotJSON(msg string) *MatrixError {
+	return &MatrixError{"M_NOT_JSON", msg}
+}
+
+// NotFound is an error when the client tries to access an unknown resource.
+func NotFound(msg string) *MatrixError {
+	return &MatrixError{"M_NOT_FOUND", msg}
+}
+
+// MissingToken is an error when the client tries to access a resource which
+// requires authentication without supplying credentials.
+func MissingToken(msg string) *MatrixError {
+	return &MatrixError{"M_MISSING_TOKEN", msg}
+}
+
+// UnknownToken is an error when the client tries to access a resource which
+// requires authentication and supplies a valid, but out-of-date token.
+func UnknownToken(msg string) *MatrixError {
+	return &MatrixError{"M_UNKNOWN_TOKEN", msg}
+}
+
+// LimitExceededError is a rate-limiting error.
+type LimitExceededError struct {
+	MatrixError
+	RetryAfterMS int64 `json:"retry_after_ms,omitempty"`
+}
+
+// LimitExceeded is an error when the client tries to send events too quickly.
+func LimitExceeded(msg string, retryAfterMS int64) *LimitExceededError {
+	return &LimitExceededError{
+		MatrixError:  MatrixError{"M_LIMIT_EXCEEDED", msg},
+		RetryAfterMS: retryAfterMS,
+	}
+}

src/github.com/matrix-org/dendrite/clientapi/jsonerror/jsonerror_test.go

+package jsonerror
+
+import (
+	"encoding/json"
+	"testing"
+)
+
+func TestLimitExceeded(t *testing.T) {
+	e := LimitExceeded("too fast", 5000)
+	jsonBytes, err := json.Marshal(&e)
+	if err != nil {
+		t.Fatalf("TestLimitExceeded: Failed to marshal LimitExceeded error. %s", err.Error())
+	}
+	want := `{"errcode":"M_LIMIT_EXCEEDED","error":"too fast","retry_after_ms":5000}`
+	if string(jsonBytes) != want {
+		t.Errorf("TestLimitExceeded: want %s, got %s", want, string(jsonBytes))
+	}
+}
+
+func TestForbidden(t *testing.T) {
+	e := Forbidden("you shall not pass")
+	jsonBytes, err := json.Marshal(&e)
+	if err != nil {
+		t.Fatalf("TestForbidden: Failed to marshal Forbidden error. %s", err.Error())
+	}
+	want := `{"errcode":"M_FORBIDDEN","error":"you shall not pass"}`
+	if string(jsonBytes) != want {
+		t.Errorf("TestForbidden: want %s, got %s", want, string(jsonBytes))
+	}
+}