Trac #32791: Human Readable Error-Message for Elliptic curve constructor.

At the moment the error-message when trying to create an elliptic curve
from a formula that defines a singular curve is not particularly user-
friendly:

{{{
sage: EllipticCurve(GF(7), [-2,0,0,0,0])
[...]
ArithmeticError: invariants (5, 0, 0, 0, 0) define a singular curve
}}}

This is trivially improvable by a one-line-fix with the tools that are
already implemented (patch at the end) to give the following much more
comprehensible error:

{{{
ArithmeticError: y^2 + 5*x*y = x^3 defines a singular curve
}}}

I wanted to create this as a pull-request, but github told me that they
don't get accepted there and I couldn't figure out where to create one
directly, so please just apply the following patch:

{{{
--- a/src/sage/schemes/elliptic_curves/ell_generic.py
+++ b/src/sage/schemes/elliptic_curves/ell_generic.py
@@ -145,7 +145,7 @@ class EllipticCurve_generic(WithEqualityById,
plane_curve.ProjectivePlaneCurve):
         self.__base_ring = K
         self.__ainvs = tuple(K(a) for a in ainvs)
         if self.discriminant() == 0:
-            raise ArithmeticError("invariants " + str(ainvs) + " define
a singular curve")
+            raise ArithmeticError(self._equation_string() + " defines a
singular curve")
         PP = projective_space.ProjectiveSpace(2, K, names='xyz')
         x, y, z = PP.coordinate_ring().gens()
         a1, a2, a3, a4, a6 = ainvs

}}}

URL: https://trac.sagemath.org/32791
Reported by: gh-Florianjw
Ticket author(s): Florian Weber
Reviewer(s): Lorenz Panny

Author: Release Manager

src/sage/doctest/forker.py

@@ -1475,7 +1475,7 @@ def report_unexpected_exception(self, out, test, example, exc_info):
             sage: old_prompt = sage0._prompt
             sage: sage0._prompt = r"\(Pdb\) "
             sage: sage0.eval("DTR.run(DT, clear_globs=False)") # indirect doctest
-            '... ArithmeticError("invariants " + str(ainvs) + " define a singular curve")'
+            '... ArithmeticError(self._equation_string() + " defines a singular curve")'
             sage: sage0.eval("l")
             '...if self.discriminant() == 0:...raise ArithmeticError...'
             sage: sage0.eval("u")

src/sage/schemes/elliptic_curves/ell_generic.py

@@ -145,7 +145,7 @@ def __init__(self, K, ainvs):
         self.__base_ring = K
         self.__ainvs = tuple(K(a) for a in ainvs)
         if self.discriminant() == 0:
-            raise ArithmeticError("invariants " + str(ainvs) + " define a singular curve")
+            raise ArithmeticError(self._equation_string() + " defines a singular curve")
         PP = projective_space.ProjectiveSpace(2, K, names='xyz')
         x, y, z = PP.coordinate_ring().gens()
         a1, a2, a3, a4, a6 = ainvs