LIVY-357. Web UI. Updated staticResourceServlet to return 404s and not throw WARNs

[ajbozarth]

LIVY-357

Since #319 added the UI whenever a static resource is accessed a MimeException is thrown as a WARN. This was caused because a InputStream must be wrapped in a BufferedInputStream for a full implementation.

I updated staticResourceServlet with the wrapper as well as adding a 404 File not found return when a file does not exist (currently an empty file is returned in such a case)

Tested manually

[ajbozarth]

@jerryshao figured out the bug you found

[jerryshao]

LGTM, thanks for the fix. CC @zjffdu .

Can you please run the travis again?

[codecov-io]

Codecov Report

Merging #330 into master will decrease coverage by 0.16%.
The diff coverage is 0%.

@@             Coverage Diff              @@
##             master     #330      +/-   ##
============================================
- Coverage     70.42%   70.26%   -0.17%     
+ Complexity      726      724       -2     
============================================
  Files            96       96              
  Lines          5123     5129       +6     
  Branches        774      777       +3     
============================================
- Hits           3608     3604       -4     
- Misses          997     1007      +10     
  Partials        518      518

Impacted Files	Coverage Δ	Complexity Δ
...in/scala/com/cloudera/livy/server/LivyServer.scala	35.5% <0%> (-1.93%)	9 <0> (ø)
...n/java/com/cloudera/livy/rsc/driver/RSCDriver.java	76.29% <0%> (-1.3%)	39% <0%> (-1%)
.../java/com/cloudera/livy/rsc/driver/JobWrapper.java	80.64% <0%> (ø)	7% <0%> (-1%)	⬇️

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[jerryshao]

Ping @zjffdu , can you please review this PR please.

[jerryshao]

Hi @ajbozarth , sorry for the delay, can you submit this PR against https://github.com/apache/incubator-livy, thanks.

[ajbozarth]

Moved to apache/incubator-livy#2