make: use '"filename" line 123' for locations instead of 'filename:123'

The format 'filename:123' is commonly used for identifying a location in
a file. Text editors recognize it and allow quick navigation through it.
The previous format was specific to make and would have required custom
support in editors.

The new format was already used in stack traces, except for the first
line. Now all lines use the same format.

branches: 1.4.2;
tests/make: replace or document .error in tests

The text 'Missing argument for ".error"' in an .exp file may be a hint
for an accidentally broken test, so eliminate them as far as possible.

tests/make: force line-based diagnostics to be listed in the tests

This way, contradictions between the intended output and the actual
output are closer together and have a better chance of being spotted.

make: add more details to debug logging of .for loops

tests/make: document and demonstrate .for i containing .if empty(i)

PR bin/43821 describes the inconsistency that in a '.for i' loop, the
condition '.if ${i:M*.c}' works since 2009 while the seemingly
equivalent condition '.if !empty(i:M*.c)' does not access the variable
'i' from the .for loop but instead the global 'i'.

Resolving this situation in a backwards-compatible and non-surprising
way is hard, as make has grown several features during the last 20 years
that interact in various edge cases. For now, document the most obvious
pitfalls.