don't panic when we can error as easily

Panicking in small helpers or in funcs that don't return error
has proved useful to keep code easier to maintain,
particularly for cases that should typically never happen.

However, in these cases we can error just as easily.
In particular, I was getting a panic whenever I forgot
that I was running garble with Go master (1.23), which is over the top.

internal/linker/linker.go

@@ -217,7 +217,7 @@ func PatchLinker(goRoot, goVersion, cacheDir, tempDir string) (string, func(), e
 
 	patchesVer, modFiles, patches, err := loadLinkerPatches(majorGoVersion)
 	if err != nil {
-		panic(fmt.Errorf("cannot retrieve linker patches: %v", err))
+		return "", nil, fmt.Errorf("cannot retrieve linker patches: %v", err)
 	}
 
 	outputLinkPath, err := cachePath(cacheDir)

main.go

@@ -200,7 +200,7 @@ func (w *uniqueLineWriter) Write(p []byte) (n int, err error) {
 		panic("unexpected use of uniqueLineWriter with -debug unset")
 	}
 	if bytes.Count(p, []byte("\n")) != 1 {
-		panic(fmt.Sprintf("log write wasn't just one line: %q", p))
+		return 0, fmt.Errorf("log write wasn't just one line: %q", p)
 	}
 	if w.seen[string(p)] {
 		return len(p), nil
@@ -1262,7 +1262,7 @@ func (tf *transformer) processImportCfg(flags []string, requiredPkgs []string) (
 		beforePath, afterPath := pair[0], pair[1]
 		lpkg, err := listPackage(tf.curPkg, beforePath)
 		if err != nil {
-			panic(err) // shouldn't happen
+			return "", err
 		}
 		if lpkg.ToObfuscate {
 			// Note that beforePath is not the canonical path.
@@ -1327,7 +1327,7 @@ func (tf *transformer) processImportCfg(flags []string, requiredPkgs []string) (
 			if strings.HasSuffix(tf.curPkg.ImportPath, ".test]") && strings.HasPrefix(tf.curPkg.ImportPath, impPath) {
 				continue
 			}
-			panic(err) // shouldn't happen
+			return "", err
 		}
 		if lpkg.Name != "main" {
 			impPath = lpkg.obfuscatedImportPath()
@@ -1470,7 +1470,7 @@ func computePkgCache(fsCache *cache.Cache, lpkg *listedPackage, pkg *types.Packa
 		// Shadowing lpkg ensures we don't use the wrong listedPackage below.
 		lpkg, err := listPackage(lpkg, imp)
 		if err != nil {
-			panic(err) // shouldn't happen
+			return computed, err
 		}
 		if lpkg.BuildID == "" {
 			continue // nothing to load

shared.go

@@ -379,7 +379,7 @@ func listPackage(from *listedPackage, path string) (*listedPackage, error) {
 			return pkg, nil
 		}
 		if listedRuntimeLinknamed {
-			panic(fmt.Sprintf("package %q still missing after go list call", path))
+			return nil, fmt.Errorf("package %q still missing after go list call", path)
 		}
 		startTime := time.Now()
 		missing := make([]string, 0, len(runtimeLinknamed))
@@ -397,11 +397,11 @@ func listPackage(from *listedPackage, path string) (*listedPackage, error) {
 		}
 		// We don't need any information about their dependencies, in this case.
 		if err := appendListedPackages(missing, false); err != nil {
-			panic(err) // should never happen
+			return nil, fmt.Errorf("failed to load missing runtime-linknamed packages: %v", err)
 		}
 		pkg, ok := sharedCache.ListedPackages[path]
 		if !ok {
-			panic(fmt.Sprintf("std listed another std package that we can't find: %s", path))
+			return nil, fmt.Errorf("std listed another std package that we can't find: %s", path)
 		}
 		listedRuntimeLinknamed = true
 		log.Printf("listed %d missing runtime-linknamed packages in %s", len(missing), debugSince(startTime))