initdb: Improve --wal-segsize handling

Give separate error messages for when the argument is not a number and
when it is not the right kind of number.

Fix wording in the help message.

src/bin/initdb/initdb.c

@@ -2323,7 +2323,7 @@ usage(const char *progname)
 	printf(_("  -U, --username=NAME       database superuser name\n"));
 	printf(_("  -W, --pwprompt            prompt for a password for the new superuser\n"));
 	printf(_("  -X, --waldir=WALDIR       location for the write-ahead log directory\n"));
-	printf(_("      --wal-segsize=SIZE    size of wal segment size in megabytes\n"));
+	printf(_("      --wal-segsize=SIZE    size of WAL segments, in megabytes\n"));
 	printf(_("\nLess commonly used options:\n"));
 	printf(_("  -d, --debug               generate lots of debugging output\n"));
 	printf(_("  -k, --data-checksums      use data page checksums\n"));
@@ -3224,11 +3224,17 @@ main(int argc, char *argv[])
 		wal_segment_size_mb = strtol(str_wal_segment_size_mb, &endptr, 10);
 
 		/* verify that wal segment size is valid */
-		if (*endptr != '\0' ||
-			!IsValidWalSegSize(wal_segment_size_mb * 1024 * 1024))
+		if (*endptr != '\0')
 		{
 			fprintf(stderr,
-					_("%s: --wal-segsize must be a power of two between 1 and 1024\n"),
+					_("%s: argument of --wal-segsize must be a number\n"),
+					progname);
+			exit(1);
+		}
+		if (!IsValidWalSegSize(wal_segment_size_mb * 1024 * 1024))
+		{
+			fprintf(stderr,
+					_("%s: argument of --wal-segsize must be a power of two between 1 and 1024\n"),
 					progname);
 			exit(1);
 		}