Improve reporting of newlocale() failures in CREATE COLLATION.

The standardized errno code for "no such locale" failures is ENOENT, which
we were just reporting at face value, viz "No such file or directory".
Per gripe from Thom Brown, this might confuse users, so add an errdetail
message to clarify what it means.  Also, report newlocale() failures as
ERRCODE_INVALID_PARAMETER_VALUE rather than using
errcode_for_file_access(), since newlocale()'s errno values aren't
necessarily tied directly to file access failures.

src/backend/utils/adt/pg_locale.c

@@ -707,8 +707,7 @@ cache_locale_time(void)
  *	otherwise returns the pointer to a static area which
  *	contains the iso formatted locale name.
  */
-static
-char *
+static char *
 IsoLocaleName(const char *winlocname)
 {
 #if (_MSC_VER >= 1400)			/* VC8.0 or later */
@@ -937,6 +936,27 @@ lc_ctype_is_c(Oid collation)
 }
 
 
+/* simple subroutine for reporting errors from newlocale() */
+static void
+report_newlocale_failure(const char *localename)
+{
+	/* copy errno in case one of the ereport auxiliary functions changes it */
+	int			save_errno = errno;
+
+	/*
+	 * ENOENT means "no such locale", not "no such file", so clarify that
+	 * errno with an errdetail message.
+	 */
+	ereport(ERROR,
+			(errcode(ERRCODE_INVALID_PARAMETER_VALUE),
+			 errmsg("could not create locale \"%s\": %m",
+					localename),
+			 (save_errno == ENOENT ?
+			  errdetail("The operating system could not find any locale data for the locale name \"%s\".",
+						localename) : 0)));
+}
+
+
 /*
  * Create a locale_t from a collation OID.	Results are cached for the
  * lifetime of the backend.  Thus, do not free the result with freelocale().
@@ -995,10 +1015,7 @@ pg_newlocale_from_collation(Oid collid)
 			result = _create_locale(LC_ALL, collcollate);
 #endif
 			if (!result)
-				ereport(ERROR,
-						(errcode_for_file_access(),
-						 errmsg("could not create locale \"%s\": %m",
-								collcollate)));
+				report_newlocale_failure(collcollate);
 		}
 		else
 		{
@@ -1008,16 +1025,10 @@ pg_newlocale_from_collation(Oid collid)
 
 			loc1 = newlocale(LC_COLLATE_MASK, collcollate, NULL);
 			if (!loc1)
-				ereport(ERROR,
-						(errcode_for_file_access(),
-						 errmsg("could not create locale \"%s\": %m",
-								collcollate)));
+				report_newlocale_failure(collcollate);
 			result = newlocale(LC_CTYPE_MASK, collctype, loc1);
 			if (!result)
-				ereport(ERROR,
-						(errcode_for_file_access(),
-						 errmsg("could not create locale \"%s\": %m",
-								collctype)));
+				report_newlocale_failure(collctype);
 #else
 
 			/*