The check for invalid paths is actually over-complicated and performed twice resulting in a performance penalty. Additionally, I decided to add unit-tests to that function.

Part of https://github.com/owncloud/core/issues/13221

I was getting a lot of these in my logs for no apparent reason, and file
uploads were failing:

	{"app":"webdav","message":"Sabre\\DAV\\Exception\\ServiceUnavailable: ","level":4,"time":"2015-01-06T15:33:39+00:00"}

In order to debug it, I had to add unique messages to all the places where
this exception was thrown, to identify which one it was, and that made the
logs much more useful:

	{"app":"webdav","message":"Sabre\\DAV\\Exception\\ServiceUnavailable: Encryption is disabled","level":4,"time":"2015-01-06T15:36:47+00:00"}

fix comments and newline

add newline

Tooltip contains 'Unable to determine date'. Fixes #6395

$userId is now a mandatory parameter for searchByTags.

Also fixed some places in the code where the argument was missing (Node
API and View)

In some cases a "NotFound" might be expected, so just return false as
before. But for other exceptions, the exception is now logged.

Local change 283c10f0

Local changes from d069ee8a
and 258ad38f

Local changes copied from ed835973
and 25370fcb

Only fixed in Local by eeee9eac

Missed in 788c8540

In some cases the 'size' field is not available resulting in some PHP errors such as:

```json
{"reqId":"03548fd9e3d3aca15a5796b3b35d7b9d","remoteAddr":"::1","app":"PHP","message":"Undefined index: size at \/Users\/lreschke\/Programming\/core\/lib\/private\/files\/fileinfo.php#125","level":3,"time":"2014-11-17T21:38:57+00:00"}
```

This can be experienced when creating a new empty file and deleting it right away, then when going to the trash bin this error is thrown.

Either we throw an exception or we ignore it. But we should certainly not print this to the end-user...

Added missing cleanPath() call that converts "/" to "" when calling
SabreDAV. This is needed because SabreDAV will discard its base URL when
passing "/".