Test included model folders missing model.config

[scpeters]

Currently there is a confusing error message if a file is loaded that finds a folder matching the name of a model to
be included but the folder does not have a model.config file. This improves the first error message and stops further loading
to prevent additional confusing messages. To illustrate this, I've added a model folder box_missing_config to test/integration/model containing a model.sdf but no model.config and a test that tries to include a model matching that folder name. Without the change to parser.cc, the test prints the following:

[ RUN      ] IncludesTest.IncludeModelMissingConfig
Error [parser.cc:749] Could not find model.config or manifest.xml for the model
Error [parser.cc:749] Could not find model.config or manifest.xml for the model
Error [parser.cc:397] File [] doesn't exist.
Error:   Could not find the 'robot' element in the xml file
         at line 80 in /build/urdfdom-YMMa9X/urdfdom-1.0.0/urdf_parser/src/model.cpp
Error [parser_urdf.cc:3193] Unable to call parseURDF on robot model
Error [parser.cc:488] parse as old deprecated model file failed.
Error Code 1 Msg: Unable to read file[]
Error Code 8 Msg: Error reading element <sdf>
/home/scpeters/clone/sdformat/test/integration/includes.cc:337: Failure
Value of: sdf::readString(stream.str(), sdfParsed)
  Actual: false
Expected: true
[  FAILED  ] IncludesTest.IncludeModelMissingConfig (51 ms)

Note that box_missing_config is not listed anywhere, which makes it hard to figure out what is the cause of the problem. It also causes readString to fail. After modifying parser.cc, the parser doesn't fail and the test shows the following console output (the second message comes from printing out the errors returned by Root::Load):

[ RUN      ] IncludesTest.IncludeModelMissingConfig
Error [parser.cc:749] Could not find model.config or manifest.xml in [/home/scpeters/clone/sdformat/test/integration/model/box_missing_config]
Error Code 12 Msg: Unable to resolve uri[box_missing_config] to model path [/home/scpeters/clone/sdformat/test/integration/model/box_missing_config] since it does not contain a model.config file.
[       OK ] IncludesTest.IncludeModelMissingConfig (34 ms)

EDIT: it doesn't actually cause sdf::readString to fail, though it does return / print an error depending on which overload is called.

[codecov-io]

Codecov Report

Merging #422 (0efbcdf) into sdf9 (0e77816) will increase coverage by 0.00%.
The diff coverage is 100.00%.

@@           Coverage Diff           @@
##             sdf9     #422   +/-   ##
=======================================
  Coverage   86.25%   86.26%           
=======================================
  Files          59       59           
  Lines        9181     9187    +6     
=======================================
+ Hits         7919     7925    +6     
  Misses       1262     1262           

Impacted Files	Coverage Δ
src/parser.cc	77.58% <100.00%> (+0.15%)	⬆️

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