Fix bugs with parsing exponential literals

Fixes https://github.com/google/closure-compiler/issues/503, in which `0E0` was treated as an invalid literal.

Also fixes the following case, which is illegal JS syntax, but throws an uncaught exception rather than printing a well formatted parsing error message.

```javascript
var x = 01E0;
```

```
java.lang.IllegalStateException: unexpected: E
	at com.google.javascript.jscomp.parsing.NewIRFactory.octaldigit(NewIRFactory.java:2218)
	at com.google.javascript.jscomp.parsing.NewIRFactory.normalizeNumber(NewIRFactory.java:2187)
	at com.google.javascript.jscomp.parsing.NewIRFactory$TransformDispatcher.processNumberLiteral(NewIRFactory.java:1366)
	at com.google.javascript.jscomp.parsing.NewIRFactory$TransformDispatcher.processNumberLiteral(NewIRFactory.java:888)
	at com.google.javascript.jscomp.parsing.NewTypeSafeDispatcher.processLiteralExpression(NewTypeSafeDispatcher.java:156)
	at com.google.javascript.jscomp.parsing.NewTypeSafeDispatcher.process(NewTypeSafeDispatcher.java:238)
	at com.google.javascript.jscomp.parsing.NewIRFactory.justTransform(NewIRFactory.java:885)
	at com.google.javascript.jscomp.parsing.NewIRFactory.transform(NewIRFactory.java:651)
	at com.google.javascript.jscomp.parsing.NewIRFactory.access$300(NewIRFactory.java:107)
	at com.google.javascript.jscomp.parsing.NewIRFactory$TransformDispatcher.processVariableDeclaration(NewIRFactory.java:1759)
	at com.google.javascript.jscomp.parsing.NewIRFactory$TransformDispatcher.processVariableDeclaration(NewIRFactory.java:888)
	at com.google.javascript.jscomp.parsing.NewTypeSafeDispatcher.process(NewTypeSafeDispatcher.java:250)
	at com.google.javascript.jscomp.parsing.NewIRFactory.justTransform(NewIRFactory.java:885)
	at com.google.javascript.jscomp.parsing.NewIRFactory.transformNodeWithInlineJsDoc(NewIRFactory.java:686)
	at com.google.javascript.jscomp.parsing.NewIRFactory.access$1800(NewIRFactory.java:107)
	at com.google.javascript.jscomp.parsing.NewIRFactory$TransformDispatcher.processVariableDeclarationList(NewIRFactory.java:1750)
	at com.google.javascript.jscomp.parsing.NewIRFactory$TransformDispatcher.processVariableDeclarationList(NewIRFactory.java:888)
	at com.google.javascript.jscomp.parsing.NewTypeSafeDispatcher.process(NewTypeSafeDispatcher.java:248)
	at com.google.javascript.jscomp.parsing.NewIRFactory.justTransform(NewIRFactory.java:885)
	at com.google.javascript.jscomp.parsing.NewIRFactory.access$3100(NewIRFactory.java:107)
	at com.google.javascript.jscomp.parsing.NewIRFactory$TransformDispatcher.processVariableStatement(NewIRFactory.java:1714)
	at com.google.javascript.jscomp.parsing.NewIRFactory$TransformDispatcher.processVariableStatement(NewIRFactory.java:888)
	at com.google.javascript.jscomp.parsing.NewTypeSafeDispatcher.process(NewTypeSafeDispatcher.java:246)
	at com.google.javascript.jscomp.parsing.NewIRFactory.justTransform(NewIRFactory.java:885)
	at com.google.javascript.jscomp.parsing.NewIRFactory.transform(NewIRFactory.java:651)
	at com.google.javascript.jscomp.parsing.NewIRFactory.access$300(NewIRFactory.java:107)
	at com.google.javascript.jscomp.parsing.NewIRFactory$TransformDispatcher.processAstRoot(NewIRFactory.java:930)
	at com.google.javascript.jscomp.parsing.NewIRFactory$TransformDispatcher.processAstRoot(NewIRFactory.java:888)
	at com.google.javascript.jscomp.parsing.NewTypeSafeDispatcher.process(NewTypeSafeDispatcher.java:236)
	at com.google.javascript.jscomp.parsing.NewIRFactory.justTransform(NewIRFactory.java:885)
	at com.google.javascript.jscomp.parsing.NewIRFactory.transformTree(NewIRFactory.java:272)
	at com.google.javascript.jscomp.parsing.ParserRunner.parseEs6(ParserRunner.java:286)
	at com.google.javascript.jscomp.parsing.ParserRunner.parse(ParserRunner.java:130)
	at com.google.javascript.jscomp.JsAst.parse(JsAst.java:85)
	at com.google.javascript.jscomp.JsAst.getAstRoot(JsAst.java:51)
	at com.google.javascript.jscomp.CompilerInput.getAstRoot(CompilerInput.java:118)
	at com.google.javascript.jscomp.Compiler.hoistExterns(Compiler.java:1453)
	at com.google.javascript.jscomp.Compiler.parseInputs(Compiler.java:1355)
	at com.google.javascript.jscomp.Compiler.parse(Compiler.java:768)
	at com.google.javascript.jscomp.Compiler.compileInternal(Compiler.java:723)
	at com.google.javascript.jscomp.Compiler.access$000(Compiler.java:94)
	at com.google.javascript.jscomp.Compiler$3.call(Compiler.java:635)
	at com.google.javascript.jscomp.Compiler$3.call(Compiler.java:632)
	at com.google.javascript.jscomp.Compiler.runInCompilerThread(Compiler.java:704)
	at com.google.javascript.jscomp.Compiler.compile(Compiler.java:632)
	at com.google.javascript.jscomp.Compiler.compile(Compiler.java:602)
```