traces: Do not print unknown location

Solves 1/3 of the infinite recursion at unknown location meme.
See #8879 for ensuring we always have a trace (for stack overflows)
We might want to re-add this for finding missing location info
*while hacking on that problem only*.

tests/lang/eval-fail-fromTOML-timestamps.err.exp

@@ -8,5 +8,3 @@ error:
             2|   key = "value"
 
        error: while parsing a TOML string: Dates and times are not supported
-
-       at «none»:0: (source not available)

tests/lang/eval-fail-hashfile-missing.err.exp

@@ -10,10 +10,6 @@ error:
 
        … while evaluating the first argument passed to builtins.toString
 
-         at «none»:0: (source not available)
-
        … while calling the 'hashFile' builtin
 
-         at «none»:0: (source not available)
-
        error: opening file '/pwd/lang/this-file-is-definitely-not-there-7392097': No such file or directory

tests/lang/eval-fail-set-override.err.exp

@@ -1,6 +1,4 @@
 error:
        … while evaluating the `__overrides` attribute
 
-         at «none»:0: (source not available)
-
        error: value is an integer while a set was expected

tests/lang/eval-fail-substring.err.exp

@@ -8,5 +8,3 @@ error:
             2|
 
        error: negative start position in 'substring'
-
-       at «none»:0: (source not available)

tests/lang/eval-fail-to-path.err.exp

@@ -9,6 +9,4 @@ error:
 
        … while evaluating the first argument passed to builtins.toPath
 
-         at «none»:0: (source not available)
-
        error: string 'foo/bar' doesn't represent an absolute path