More robust handling of AST validation

[AlexNisnevich]

(Fixes #257)

• Introduces ErrorAnalyzer.bestValidParse(), which wraps SemanticParseResult.bestParse and additionally prefers parses that pass the AstValidator, if any (this resolves issues where the "best" parse fails the AstValidator but there is actually a passing parse further down)
• Use ErrorAnalyzer.bestValidParse everywhere we run the parser (server, console, tests)
• Treat ConditionalAction the same as TriggeredAbility for certain AST validation rules (fixes an edge-case bug caught by test suite where "When this robot is destroyed, destroy a robot" has two valid parses, one as TriggeredAbility and one as ConditionalAction)
• Parse "if this robot is not adjacent to an object" correctly (returning a parse that doesn't violate AST validation)