Improve diagnostics for missing source code file

[alexander-yevsyukov]

I accidentally use wrong path for a source code file with fences and got the following console output:

2026/02/16 18:04:03 INFO starting application, reading args...
panic: embedding error for file ``: unable to embed construction for doc file `../content/docs/validation/01-getting-started/first-model.md` at line 110: unable to read the code fence end: file /Users/sanders/Projects/Spine/validation/docs/_bin/build/fragments/first-model/src/test/kotlin/io/spine/validation/docs/firstmodel/BankCardKtTest-51ad477f.java doesn't exist.

goroutine 1 [running]:
embed-code/embed-code-go/embedding.EmbedAll({{0x1400000e810, 0x8}, {0x1400000e830, 0x10}, {0x10494a6c0, 0x1, 0x1}, {0x10494bc40, 0x2, 0x2}, ...})
        /Users/vladyslav.kuksiuk/Desktop/Work/embed-code/embed-code-go/embedding/processor.go:134 +0x178
embed-code/embed-code-go/cli.EmbedCodeSamples({{0x1400000e810, 0x8}, {0x1400000e830, 0x10}, {0x10494a6c0, 0x1, 0x1}, {0x10494bc40, 0x2, 0x2}, ...})
        /Users/vladyslav.kuksiuk/Desktop/Work/embed-code/embed-code-go/cli/cli.go:111 +0x70
main.main()
        /Users/vladyslav.kuksiuk/Desktop/Work/embed-code/embed-code-go/main.go:111 +0x48c

Here are the problems with this output.

Empty file name reference

Why don't we have anything between backticks in the piece below?

panic: embedding error for file ``:

Hash in the name of the file

The file name I wrongly used was BankCardKtTest.java. It should have been BankCardKtTest.kt. But why do we have 51ad477f in the below fragment:

BankCardKtTest-51ad477f.java doesn't exist.

Stacktrace is redundant for regular users

The below block has very limited value for regular users of the tool:

goroutine 1 [running]:
embed-code/embed-code-go/embedding.EmbedAll({{0x1400000e810, 0x8}, {0x1400000e830, 0x10}, {0x10494a6c0, 0x1, 0x1}, {0x10494bc40, 0x2, 0x2}, ...})
        /Users/vladyslav.kuksiuk/Desktop/Work/embed-code/embed-code-go/embedding/processor.go:134 +0x178
embed-code/embed-code-go/cli.EmbedCodeSamples({{0x1400000e810, 0x8}, {0x1400000e830, 0x10}, {0x10494a6c0, 0x1, 0x1}, {0x10494bc40, 0x2, 0x2}, ...})
        /Users/vladyslav.kuksiuk/Desktop/Work/embed-code/embed-code-go/cli/cli.go:111 +0x70
main.main()
        /Users/vladyslav.kuksiuk/Desktop/Work/embed-code/embed-code-go/main.go:111 +0x48c

It would be good to have stacktrace only when the utility is executed with the --stacktrace flag.

References to the developer's file system in stacktrace

Why do we have the path /Users/vladyslav.kuksiuk/Desktop/Work in stacktrace? Can we have the path to the GitHub instead?