Rearrange libpq's error reporting to avoid duplicated error text.

Since commit ffa2e4670, libpq accumulates text in conn->errorMessage
across a whole query cycle.  In some situations, we may report more
than one error event within a cycle: the easiest case to reach is
where we report a FATAL error message from the server, and then a
bit later we detect loss of connection.  Since, historically, each
error PGresult bears the entire content of conn->errorMessage,
this results in duplication of the FATAL message in any output that
concatenates the contents of the PGresults.

Accumulation in errorMessage still seems like a good idea, especially
in view of the number of places that did ad-hoc error concatenation
before ffa2e4670.  So to fix this, let's track how much of
conn->errorMessage has been read out into error PGresults, and only
include new text in later PGresults.  The tricky part of that is
to be sure that we never discard an error PGresult once made (else
we'd risk dropping some text, a problem much worse than duplication).
While libpq formerly did that in some code paths, a little bit of
rearrangement lets us postpone making an error PGresult at all until
we are about to return it.

A side benefit of that postponement is that it now becomes practical
to return a dummy static PGresult in cases where we hit out-of-memory
while trying to manufacture an error PGresult.  This eliminates the
admittedly-very-rare case where we'd return NULL from PQgetResult,
indicating successful query completion, even though what actually
happened was an OOM failure.

Discussion: https://postgr.es/m/ab4288f8-be5c-57fb-2400-e3e857f53e46@enterprisedb.com

src/interfaces/libpq/fe-connect.c

@@ -3685,7 +3685,7 @@ keep_going:						/* We will come back to here until there is
 				 * (and it seems some clients expect it to be empty after a
 				 * successful connection).
 				 */
-				resetPQExpBuffer(&conn->errorMessage);
+				pqClearConnErrorState(conn);
 
 				/* We are open for business! */
 				conn->status = CONNECTION_OK;
@@ -4231,7 +4231,7 @@ closePGconn(PGconn *conn)
 
 	/*
 	 * Close the connection, reset all transient state, flush I/O buffers.
-	 * Note that this includes clearing conn->errorMessage; we're no longer
+	 * Note that this includes clearing conn's error state; we're no longer
 	 * interested in any failures associated with the old connection, and we
 	 * want a clean slate for any new connection attempt.
 	 */
@@ -4241,7 +4241,7 @@ closePGconn(PGconn *conn)
 	conn->xactStatus = PQTRANS_IDLE;
 	conn->pipelineStatus = PQ_PIPELINE_OFF;
 	pqClearAsyncResult(conn);	/* deallocate result */
-	resetPQExpBuffer(&conn->errorMessage);
+	pqClearConnErrorState(conn);
 	release_conn_addrinfo(conn);
 
 	/* Reset all state obtained from server, too */
@@ -5236,7 +5236,7 @@ ldapServiceLookup(const char *purl, PQconninfoOption *options,
  * Returns 0 on success, nonzero on failure.  On failure, if errorMessage
  * isn't null, also store an error message there.  (Note: the only reason
  * this function and related ones don't dump core on errorMessage == NULL
- * is the undocumented fact that printfPQExpBuffer does nothing when passed
+ * is the undocumented fact that appendPQExpBuffer does nothing when passed
  * a null PQExpBuffer pointer.)
  */
 static int