Add Steve Johnson-style parser recovery rules:

SourceUnit = Error $
    Block = '{' Error '}'
    ContractDefinition = '{' Error '}'
    Statement = Error ';'

Co-Authored-By: chriseth <chris@ethereum.org>

test/libsolidity/errorRecoveryTests/missing_rhs.sol

@@ -0,0 +1,11 @@
+pragma solidity >=0.0.0;
+
+contract Error3 {
+	constructor() public {
+	    balances[tx.origin] = ; // missing RHS.
+	}
+
+}
+// ----
+// ParserError: (95-96): Expected primary expression.
+// Warning: (95-96): Recovered in Statement at ';'.