Improve error messages in "java.so -jar"

Print useful error messages, instead of cryptic exception traces,
in three cases of "java.so -jar something.jar":

 1. When something.jar doesn't exist
 2. When something.jar exists, but can't be read as a jar (zip) file
 3. When something.jar exists, but doesn't have a "Main-Class" field
    in its manifest.