[bugfix] Fail properly when the source code language cannot be guessed

[teojgo]

• The SingleSource build system now raises an exception when
  it fails to guess the programming language of the source file.

• Add unittests.

Fixes #1546

[codecov-io]

Codecov Report

Merging #1619 (699899e) into master (e8f9253) will increase coverage by 0.01%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master    #1619      +/-   ##
==========================================
+ Coverage   87.66%   87.68%   +0.01%     
==========================================
  Files          44       44              
  Lines        7283     7283              
==========================================
+ Hits         6385     6386       +1     
+ Misses        898      897       -1     

Impacted Files	Coverage Δ
reframe/core/buildsystems.py	97.47% <100.00%> (+0.36%)	⬆️

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[vkarak]

What is not so clear to me is how this was triggering the bug that we observed. Can you explain a bit?

[teojgo]

What is not so clear to me is how this was triggering the bug that we observed. Can you explain a bit?

So in our case, even though the sourcepath was non existent, the build system did not raise the exception during autodetection. Therefore, no actual compilation/build command was generated and the compilation script did nothing so it was not failing.

[vkarak]

lgtm

[vkarak]

@jenkins-cscs retry dom