[SPARK-4107] Fix incorrect handling of read() and skip() return values

`read()` may return fewer bytes than requested; when this occurred, the old code would silently return less data than requested, which might cause stream corruption errors.  `skip()` faces similar issues, too.

This patch fixes several cases where we mis-handle these methods' return values.

Author: Josh Rosen <joshrosen@databricks.com>

Closes #2969 from JoshRosen/file-channel-read-fix and squashes the following commits:

e724a9f [Josh Rosen] Fix similar issue of not checking skip() return value.
cbc03ce [Josh Rosen] Update the other log message, too.
01e6015 [Josh Rosen] file.getName -> file.getAbsolutePath
d961d95 [Josh Rosen] Fix another issue in FileServerSuite.
b9265d2 [Josh Rosen] Fix a similar (minor) issue in TestUtils.
cd9d76f [Josh Rosen] Fix a similar error in Tachyon:
3db0008 [Josh Rosen] Fix a similar read() error in Utils.offsetBytes().
db985ed [Josh Rosen] Fix unsafe usage of FileChannel.read():