Fix exceptions thrown by FGFunction and caught by FGAerodynamics

[bcoconni]

The class FGAerodynamics currently catches std::string exceptions thrown by the constructor of FGFunction. However FGFunction is now throwing a mix of exceptions deriving from both std::string and JSBSim::BaseException, so FGAerodynamics is no longer catching all exceptions thrown by the constructor of FGFunction.

This PR is changing:

• the remaining std::string exceptions in FGFunction to JSBSim::BaseException
• the format of exceptions caught by FGAerodynamics from std::string to JSBSim::BaseException.

Also the try/catch block in FGAerodynamics is simplified in the process. Currently the code looks like:

if (condition) {
  try {
    function1 = new FGFunction();
  }
  catch (const string& str) {
    log << "Fatal error: " << str;
  }
}
else {
  try {
    function2 = new FGFunction();
  }
  catch (const string& str) {
    log << "Fatal error: " << str;
  }
}

This PR avoids the duplication of the try/catch block by moving the if/else condition into the try block:

try {
  if (condition) {
    function1 = new FGFunction();
  } else {
    function2 = new FGFunction();
  }
}
catch (const BaseException& e) {
    log << "Fatal error: " << e.what();
}

[codecov]

Codecov Report

Attention: Patch coverage is 0% with 20 lines in your changes missing coverage. Please review.

Project coverage is 25.13%. Comparing base (e607c27) to head (aa97d04).
Report is 1 commits behind head on master.

Files with missing lines	Patch %	Lines
src/math/FGFunction.cpp	0.00%	13 Missing ⚠️
src/models/FGAerodynamics.cpp	0.00%	7 Missing ⚠️

Additional details and impacted files

@@           Coverage Diff           @@
##           master    #1152   +/-   ##
=======================================
  Coverage   25.12%   25.13%           
=======================================
  Files         170      170           
  Lines       18337    18330    -7     
=======================================
  Hits         4608     4608           
+ Misses      13729    13722    -7     

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[seanmcleod]

Looks good.