net/url: improve url parsing error messages by quoting

Current implementation doesn't always make it obvious what the exact
problem with the URL is, so this makes it clearer by consistently quoting
the invalid URL, as is the norm in other parsing implementations, eg.:
strconv.Atoi(" 123") returns an error: parsing " 123": invalid syntax

Updates golang#29261

src/net/http/readrequest_test.go

@@ -133,7 +133,7 @@ var reqTests = []reqTest{
 		nil,
 		noBodyStr,
 		noTrailer,
-		"parse ../../../../etc/passwd: invalid URI for request",
+		"parse \"../../../../etc/passwd\": invalid URI for request",
 	},
 
 	// Tests missing URL:
@@ -143,7 +143,7 @@ var reqTests = []reqTest{
 		nil,
 		noBodyStr,
 		noTrailer,
-		"parse : empty url",
+		"parse \"\": empty url",
 	},
 
 	// Tests chunked body with trailer:

src/net/url/url.go

@@ -481,13 +481,13 @@ func Parse(rawurl string) (*URL, error) {
 	u, frag := split(rawurl, "#", true)
 	url, err := parse(u, false)
 	if err != nil {
-		return nil, &Error{"parse", u, err}
+		return nil, &Error{"parse", strconv.Quote(u), err}
 	}
 	if frag == "" {
 		return url, nil
 	}
 	if url.Fragment, err = unescape(frag, encodeFragment); err != nil {
-		return nil, &Error{"parse", rawurl, err}
+		return nil, &Error{"parse", strconv.Quote(rawurl), err}
 	}
 	return url, nil
 }
@@ -500,7 +500,7 @@ func Parse(rawurl string) (*URL, error) {
 func ParseRequestURI(rawurl string) (*URL, error) {
 	url, err := parse(rawurl, true)
 	if err != nil {
-		return nil, &Error{"parse", rawurl, err}
+		return nil, &Error{"parse", strconv.Quote(rawurl), err}
 	}
 	return url, nil
 }