Settled on method for dealing with errors originating from within the

R interpreter. Added global string "last_R_error_msg" to hold the result
of geterrmessage(). An error in R now calls throw_r_error() with the
result of geterrmessage() as its argument. throw_r_error() saves argument
in last_R_error_msg. When the R call ends, PL/R will elog using
last_R_error_msg whenever it is set, or a generic message when it is not.
This works the same for both 7.3 and 7.4.

pg_rsupport.c

@@ -33,6 +33,7 @@
 #include "plr.h"
 
 extern MemoryContext plr_SPI_context;
+extern char *last_R_error_msg;
 
 static SEXP rpgsql_get_results(int ntuples, SPITupleTable *tuptable);
 
@@ -593,30 +594,20 @@ plr_SPI_lastoid(void)
 	return result;
 }
 
-#ifdef PG_VERSION_73_COMPAT
-/*************************************************************************
- * working with postgres 7.3 compatible sources
- *************************************************************************/
 void
 throw_r_error(const char **msg)
 {
-	throw_pg_notice(msg);
+	if (msg && *msg)
+		last_R_error_msg = pstrdup(*msg);
+	else
+		last_R_error_msg = pstrdup("caught error calling R function");
 }
 
-#else
+#ifndef PG_VERSION_73_COMPAT
 /*************************************************************************
  * working with postgres 7.4 compatible sources
  *************************************************************************/
 
-void
-throw_r_error(const char **msg)
-{
-	if (msg && *msg)
-		elog(ERROR, "%s", *msg);
-	else
-		elog(ERROR, "%s", "");
-}
-
 /*
  * error context callback to let us supply a call-stack traceback
  */

plr.c

@@ -37,6 +37,7 @@
  */
 MemoryContext plr_SPI_context;
 HTAB *plr_HashTable = (HTAB *) NULL;
+char *last_R_error_msg = NULL;
 
 static bool plr_firstcall = true;
 static bool	plr_interp_started = false;
@@ -182,15 +183,23 @@ load_r_cmd(const char *cmd)
 	PROTECT(cmdexpr = R_ParseVector(cmdSexp, -1, &status));
 	if (status != PARSE_OK) {
 	    UNPROTECT(2);
-	    elog(ERROR, "plr: invalid R call: %s", cmd);
+		if (last_R_error_msg)
+			elog(ERROR, "%s", last_R_error_msg);
+		else
+			elog(ERROR, "R parse error: %s", cmd);
 	}
 
 	/* Loop is needed here as EXPSEXP may be of length > 1 */
 	for(i = 0; i < length(cmdexpr); i++)
 	{
 		ans = R_tryEval(VECTOR_ELT(cmdexpr, i), R_GlobalEnv, &status);
 		if(status != 0)
-			elog(ERROR, "Caught an error calling R function");
+		{
+			if (last_R_error_msg)
+				elog(ERROR, "%s", last_R_error_msg);
+			else
+				elog(ERROR, "%s", "caught error calling R function");
+		}
 	}
 
 	UNPROTECT(2);
@@ -836,7 +845,10 @@ plr_parse_func_body(const char *body)
 	if (status != PARSE_OK)
 	{
 	    UNPROTECT(2);
-		elog(ERROR, "plr: R parse error");
+		if (last_R_error_msg)
+			elog(ERROR, "%s", last_R_error_msg);
+		else
+			elog(ERROR, "R parse error in function body");
 	}
 
 	UNPROTECT(2);
@@ -875,8 +887,12 @@ call_r_func(SEXP fun, SEXP rargs)
 	UNPROTECT(1);
 
 	if(errorOccurred)
-		elog(ERROR, "Caught an error calling R function");
-
+	{
+		if (last_R_error_msg)
+			elog(ERROR, "%s", last_R_error_msg);
+		else
+			elog(ERROR, "%s", "caught error calling R function");
+	}
 	return ans;
 }