Bug: readJJM() fails early when outputs are missing

[jimianelli]

In R/jjmr-internal.R, .getOutFile() uses normalizePath() with mustWork=TRUE by default. If outputs are missing, readJJM() fails before a clear error can be issued.

Proposed fix:

• normalizePath(..., mustWork = FALSE)
• Add explicit file.exists() checks in readJJM() for .yld/.par with targeted messages.