handle invalid replacements more gracefully

...e.r.nodes/src/com/oracle/truffle/r/nodes/RASTBuilder.java

...c/com/oracle/truffle/r/nodes/control/ReplacementNode.java

...le.r.runtime/src/com/oracle/truffle/r/runtime/RError.java

...est/src/com/oracle/truffle/r/test/ExpectedTestOutput.test

.../truffle/r/test/builtins/TestBuiltin_parentenvassign.java

...oracle/truffle/r/test/library/base/TestSimpleParsing.java