Error rendering shared by the CLI and the rebar3 plugin.

Located problems — lexer/parser errors and the semantic pass findings, both carrying a file path and a location — render in the rebar3/elp style: a source excerpt with a gutter and a ╰── marker naming the problem. Locations may carry a column ({Line, Column} pairs thread through from the lexer); the marker then sits on the exact column, counted in codepoints of the tab-expanded line, falling back to the first non-blank column for line-only locations. When the source cannot be read back, carries no such line, or the error names no location at all, the compact path:line[:column]: message form answers instead. The source lookup is injectable — tests and the --compact-errors switch feed a resolver that always errors.