move ConversionFailedException into separate class, remove unused ConvertNode

...in/src/com/oracle/truffle/r/nodes/builtin/base/NChar.java

...n/src/com/oracle/truffle/r/nodes/builtin/base/NZChar.java

...src/com/oracle/truffle/r/nodes/unary/CastComplexNode.java

.../src/com/oracle/truffle/r/nodes/unary/CastDoubleNode.java

...src/com/oracle/truffle/r/nodes/unary/CastIntegerNode.java

...src/com/oracle/truffle/r/nodes/unary/CastLogicalNode.java

...des/src/com/oracle/truffle/r/nodes/unary/CastRawNode.java

...acle/truffle/r/nodes/unary/ConversionFailedException.java

...odes/src/com/oracle/truffle/r/nodes/unary/ConvertInt.java