Improve plist handling for hdiutil commands

[rolandwalker]

• Raise CaskError instead of Plist::ParseError from module
• Improve error message when parse result is empty
• remove leading garbage text and emit it to stderr (seen in Error: undefined method `ParseError' for Plist:Module #5060)
• remove trailing garbage text and emit it to stderr (seen in install google-chrome failed #4819)

This has the incidental effect of emitting DMG licenses during
installation, which seems desirable as permanent functionality.

If not permanent, the warnings to STDERR should still be kept
temporarily to help get better bug reports on hdiutil.

A bug wrt DMG licenses must have been introduced in one of
#4892, #4887, #4889, #4900, #4975, #4978, or #4857. Presumably,

the cause is that STDERR was previously silenced when running
hdiutil. It would be cleaner (and more reliable) to redirect
STDERR and examine it separately, rather than clean up the
merged outputs.

closes #4819
closes #5060

[vitorgalvao]

This has the incidental effect of emitting DMG licenses during installation, which seems desirable as permanent functionality.

As @alebcay pointed out in #5191, the message is “a bit startling, probably especially so to new users”. I agree. If even contributors were left to wonder what that was (I also encountered that recently, with Charles), I imagine many users will as well, specially since this does not happen with every cask. I also worry it will have the adverse effect of making users ignore important messages in other casks (i.e., the caveats): after seeing a bunch of text they don’t care about, a few times, they’ll be more prone to ignore future messages.

I’d also like to revive the discussion on dmg agreements. My opinion has not changed: I still believe we should not install apps without the user agreeing, unless a specific option is set.

[rolandwalker]

OK, I will remove this accidental "feature".

Though we aren't entirely on the same page wrt DMG agreements, I'm open to your argument.

However, one thing we cannot do is stop and interactively query as hdiutil likes to do. We should revisit DMG agreements once we have a persistent config file.

[vitorgalvao]

I would very much like your opinion on the issue. Particularly now, that we’ve already decided the fate of one cask based on its legality, I find that accepting legal agreements on behalf of the user by default is not something we should be doing.