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/multiple_errors.sol

@@ -0,0 +1,29 @@
+// An example with multiple errors.
+// Most are caught by inserting an expected token.
+// However some us S C Johnson recovery to
+// skip over tokens.
+
+pragma solidity >=0.0.0;
+
+contract Error4 {
+  constructor() public {
+    balances[tx.origin] = 1 2; // missing operator
+  }
+
+  function sendCoin(address receiver, uint amount) public returns(bool sufficient) {
+    if (balances[msg.sender] < amount) return false;
+    balances[msg.sender] -= amount   // Missing ";"
+    balances[receiver] += amount   // Another missing ";"
+    emit Transfer(msg.sender  // truncated line
+    return true;
+  }
+
+
+}
+// ----
+// ParserError: (249-250): Expected ';' but got 'Number'
+// ParserError: (471-479): Expected ';' but got identifier
+// ParserError: (529-533): Expected ';' but got 'emit'
+// ParserError: (577-583): Expected ',' but got 'return'
+// ParserError: (577-583): Expected primary expression.
+// Warning: (588-589): Recovered in Statement at ';'.