Better error handling and other fixes

• Fix indentation
• Exit from 'run' script if compilation fails
• Rename methods from 'canBe' to 'is'
• Rename method 'codegen' to 'translate'
• Rework error management
• Rename terminal symbol 'LITNUM' to 'LITREAL'
• Change type of semantic value of 'LITREAL'
• Allow scientific notation in 'real' literals
• Update documentation