Revert "Improve error message on snapshot import in snapmgr.c"

This reverts commit a0d87bcd9b, following a remark from Andres Frend
that the new error can be triggered with an incorrect SET TRANSACTION
SNAPSHOT command without being really helpful for the user as it uses
the internal file name.

Discussion: https://postgr.es/m/20230914020724.hlks7vunitvtbbz4@awork3.anarazel.de
Backpatch-through: 11

src/backend/utils/time/snapmgr.c

@@ -1540,9 +1540,8 @@ ImportSnapshot(const char *idstr)
 	f = AllocateFile(path, PG_BINARY_R);
 	if (!f)
 		ereport(ERROR,
-				(errcode_for_file_access(),
-				 errmsg("could not open file \"%s\" for reading: %m",
-						path)));
+				(errcode(ERRCODE_INVALID_PARAMETER_VALUE),
+				 errmsg("invalid snapshot identifier: \"%s\"", idstr)));
 
 	/* get the size of the file so that we know how much memory we need */
 	if (fstat(fileno(f), &stat_buf))