More informative error message. (#419)

### TL;DR

- Added more descriptive error message for `JACOBIAN_75_TIMES_BAD` which now explains that this indicates self-intersecting flux surfaces
- Simplified the `VmecStatus` enum in `util.h` by removing several unused status codes

### Why make this change?

The previous error messages were terse and didn't provide enough context to understand what went wrong during VMEC execution. These improvements make debugging failed VMEC runs easier by providing more descriptive information about the nature of the failures, especially for common issues like overly shaped boundaries causing self-intersecting flux surfaces.

Author: jurasic-pf

src/vmecpp/cpp/vmecpp/common/util/util.cc

@@ -27,26 +27,19 @@ std::string VmecStatusAsString(const VmecStatus vmec_status) {
       return "NORMAL_TERMINATION";
     case VmecStatus::BAD_JACOBIAN:
       return "BAD_JACOBIAN";
-    case VmecStatus::NCURR_NE_1_BLOAT_NE_1:
-      return "NCURR_NE_1_BLOAT_NE_1";
     case VmecStatus::JACOBIAN_75_TIMES_BAD:
-      return "JACOBIAN_75_TIMES_BAD";
-    case VmecStatus::INPUT_PARSING_ERROR:
-      return "INPUT_PARSING_ERROR";
-    case VmecStatus::PHIEDGE_WRONG_SIGN:
-      return "PHIEDGE_WRONG_SIGN";
-    case VmecStatus::NS_ERROR:
-      return "NS_ERROR";
-    case VmecStatus::MISC_ERROR:
-      return "MISC_ERROR";
-    case VmecStatus::VAC_VMEC_ITOR_MISMATCH:
-      return "VAC_VMEC_ITOR_MISMATCH";
+      return "JACOBIAN_75_TIMES_BAD The jacobian factor of the geometry "
+             "repeatedly "
+             "became negative, indicating that flux surfaces become "
+             "self-intersecting. This "
+             "can mean that your prescribed boundary is too shaped for VMEC to "
+             "resolve ";
     case VmecStatus::SUCCESSFUL_TERMINATION:
       return "SUCCESSFUL_TERMINATION";
   }
 
   // never reached
-  return "UNKNOWN??";
+  return "UNKNOWN??" + std::to_string(static_cast<int>(vmec_status));
 }
 
 // -1 if x<0, 0 if x==0, +1 if x>0

src/vmecpp/cpp/vmecpp/common/util/util.h

@@ -149,15 +149,7 @@ enum class VmecStatus : std::uint8_t {
   // no fatal error but convergence was not reached
   NORMAL_TERMINATION = 0,
   BAD_JACOBIAN = 1,
-  // 2 TODO(jons): not used anymore?
-  NCURR_NE_1_BLOAT_NE_1 = 3,
   JACOBIAN_75_TIMES_BAD = 4,
-  INPUT_PARSING_ERROR = 5,
-  // 6 TODO(jons): not used anymore?
-  PHIEDGE_WRONG_SIGN = 7,
-  NS_ERROR = 8,    // NS ARRAY MUST NOT BE ALL ZEROES
-  MISC_ERROR = 9,  // can happen in mgrid_mod
-  VAC_VMEC_ITOR_MISMATCH = 10,
   // everything went well, VMEC++ converged
   SUCCESSFUL_TERMINATION = 11
 };

src/vmecpp/cpp/vmecpp/vmec/vmec/vmec.cc

@@ -325,8 +325,8 @@ absl::StatusOr<bool> Vmec::run(const VmecCheckpoint& checkpoint,
       if (status_ != VmecStatus::NORMAL_TERMINATION &&
           status_ != VmecStatus::SUCCESSFUL_TERMINATION) {
         const auto msg = absl::StrFormat(
-            "FATAL ERROR in SolveEquilibrium.\nVmec status "
-            "code: %s\nVmecINDATA had these contents:\n%s",
+            "FATAL ERROR in SolveEquilibrium: %s\n"
+            "VmecINDATA had these contents:\n%s",
             VmecStatusAsString(status_), *indata_.ToJson());
 
         return absl::InternalError(msg);