Catch solver exceptions

Problem

Providing an input file that results in a singular system matrix (e.g., through missing connections) will throw an error like

Assertion failed: (m_factorizationIsOk && "The matrix should be factorized first"), function _solve_impl, file SparseLU.h, line 340.

This is thrown in this line:
https://github.com/SimVascular/svZeroDSolver/blob/32b46d7ba376c8384842902aadc45aeb6d875bbd/src/algebra/SparseSystem.cpp#L104

This is a very generic error by Eigen/SuperLU and doesn't help the user find what is wrong with the input file.

Solution

Throw a helpful error message and tell the user what is wrong with the input file.

Additional context

We've had this problem before, e.g., #94

Code of Conduct

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Start at src/algebra/SparseSystem.cpp line 104 and review the solver path that triggers Eigen/SuperLU's singular-matrix assertion. Reproduce the failure with an input containing missing connections, and consult issue #94 for related context. Done means the solver reports a helpful error that identifies the input problem instead of exposing the generic assertion.