Improve error handling and debugging

[farblos]

Improve previous password entry changes

OraclePatchDownloader.java:

• Factor out common code from methods readLine and readPassword to a new method readThing. Protect from reading EOF in that method.

• Change syntax for reading passwords from environment to a more explicit env:ENV_VAR.

• Simplify calls to readLine, readPrompt.

README.md:

• Sync online help.

• Change example to the env:ENV_VAR syntax.

Clean up code

OraclePatchDownloader.java:

• Reorder definitions of global variables to be more consistent with rest of code.

• Make variable tempdirDelete local to main method, renamed patchRegexp to patternList.

• Completely remove unused code referring to checkPatchList. (That variable never gets set to true as of commit 7aaba1e.)

• Consistently use "private static" methods, make the constructor a "public static" method.

• Check for empty download list after processing all search results.

• Move exception handling from method download to main to avoid exiting prematurely (thus circumventing clean-up code in main).

• Simplify handling of patterns in command line processing.

Streamline error handling

OraclePatchDownloader.java:

• Add methods format(), error(), warn(), usage() as main APIs to handle errors and use them as appropriate. When none of these is appropriate, throw a RuntimeExcpetion. Extend main method to handle any ExitExceptions thrown by the new methods.

• Redo web scraping to use methods that throw ElementNotFoundExceptions when a page does not look as expected. Handle these exceptions.

• Simplify download link loop by using an XPath query.

• Turn "no dowmloads error" into a warning (since this is a legitimate use case, not an error).

• Use try-with-resource where appropriate.

[farblos]

Another fat PR ... at least I split it up into separate commits this time. Please check and let me know what you think.

This was (hopefully) the last "infrastructure" work, so the following commits/PRs should be more focused.

I will try to monitor the issues, just in case any of these changes breaks things. In any case, feel free to CC my as @farblos if there should be need for it!