Improve "invalid byte sequence for encoding" message

It used to say

ERROR:  invalid byte sequence for encoding "UTF8": 0xdb24

Change this to

ERROR:  invalid byte sequence for encoding "UTF8": 0xdb 0x24

to make it clear that this is a byte sequence and not a code point.

Also fix the adjacent "character has no equivalent" message that has
the same issue.

src/backend/utils/mb/wchar.c

@@ -1595,7 +1595,7 @@ void
 report_invalid_encoding(int encoding, const char *mbstr, int len)
 {
 	int			l = pg_encoding_mblen(encoding, mbstr);
-	char		buf[8 * 2 + 1];
+	char		buf[8 * 5 + 1];
 	char	   *p = buf;
 	int			j,
 				jlimit;
@@ -1604,11 +1604,15 @@ report_invalid_encoding(int encoding, const char *mbstr, int len)
 	jlimit = Min(jlimit, 8);	/* prevent buffer overrun */
 
 	for (j = 0; j < jlimit; j++)
-		p += sprintf(p, "%02x", (unsigned char) mbstr[j]);
+	{
+		p += sprintf(p, "0x%02x", (unsigned char) mbstr[j]);
+		if (j < jlimit - 1)
+			p += sprintf(p, " ");
+	}
 
 	ereport(ERROR,
 			(errcode(ERRCODE_CHARACTER_NOT_IN_REPERTOIRE),
-			 errmsg("invalid byte sequence for encoding \"%s\": 0x%s",
+			 errmsg("invalid byte sequence for encoding \"%s\": %s",
 					pg_enc2name_tbl[encoding].name,
 					buf)));
 }
@@ -1624,7 +1628,7 @@ report_untranslatable_char(int src_encoding, int dest_encoding,
 						   const char *mbstr, int len)
 {
 	int			l = pg_encoding_mblen(src_encoding, mbstr);
-	char		buf[8 * 2 + 1];
+	char		buf[8 * 5 + 1];
 	char	   *p = buf;
 	int			j,
 				jlimit;
@@ -1633,11 +1637,15 @@ report_untranslatable_char(int src_encoding, int dest_encoding,
 	jlimit = Min(jlimit, 8);	/* prevent buffer overrun */
 
 	for (j = 0; j < jlimit; j++)
-		p += sprintf(p, "%02x", (unsigned char) mbstr[j]);
+	{
+		p += sprintf(p, "0x%02x", (unsigned char) mbstr[j]);
+		if (j < jlimit - 1)
+			p += sprintf(p, " ");
+	}
 
 	ereport(ERROR,
 			(errcode(ERRCODE_UNTRANSLATABLE_CHARACTER),
-	  errmsg("character 0x%s of encoding \"%s\" has no equivalent in \"%s\"",
+	  errmsg("character with byte sequence %s in encoding \"%s\" has no equivalent in encoding \"%s\"",
 			 buf,
 			 pg_enc2name_tbl[src_encoding].name,
 			 pg_enc2name_tbl[dest_encoding].name)));