feat: use warning and not exception to indicate no convergence

structuralcodes/__init__.py

@@ -1,7 +1,10 @@
 """A Python package that contains models from structural design codes."""
 
+import warnings
+
 from . import codes, core, geometry, materials, sections
 from .codes import get_design_codes, set_design_code, set_national_annex
+from .core.errors import StructuralCodesWarning
 
 __version__ = '0.6.1'
 
@@ -15,3 +18,5 @@
     'geometry',
     'sections',
 ]
+
+warnings.filterwarnings(action='always', category=StructuralCodesWarning)

structuralcodes/core/errors.py

@@ -0,0 +1,11 @@
+"""General exception and warning classes."""
+
+
+class StructuralCodesWarning(Warning):
+    """Base class for StructurlCodes warnings."""
+
+
+class NoConvergenceWarning(StructuralCodesWarning):
+    """A warning that indicates that no convergence was reached for an
+    iterative solver.
+    """